Add minimal documentation of wxPropertyGrid XRC format

Not everything is documented yet, mostly because it's not really easy to
find out what "everything" actually is.

docs/doxygen/overviews/propgrid.h

@@ -578,9 +578,8 @@ in an arbitrary list of wxVariants.
 @subsection propgrid_fromfile Loading Population from a Text-based Storage
 
 Class wxPropertyGridPopulator may be helpful when writing code that
-loads properties from a text-source. In fact, the wxPropertyGrid xrc-handler
+loads properties from a text-source. It is used by wxPropertyGrid XRC handler,
-(which may not be currently included in wxWidgets, but probably will be in
+for example.
-near future) uses it.
 
 @subsection editablestate Saving and Restoring User-Editable State
 

docs/doxygen/overviews/xrc_format.h

@@ -298,6 +298,13 @@ DPI-dependent scaling, i.e. the format is just
 and @c x and @c y are just integers which are not interpreted in any way.
 
 
+@subsection overview_xrcformat_type_percent Integer or percent
+
+This is a value which can be either a simple (unsigned) integer or a percent,
+specified as `N%`, with a literal percent sign, of some other implicitly
+specified length.
+
+
 @subsection overview_xrcformat_type_text Text
 
 String properties use several escape sequences that are translated according to
@@ -1737,6 +1744,134 @@ wxPanel may have optional children: either exactly one
 objects.
 
 
+@subsubsection xrc_wxpropertygrid wxPropertyGrid
+
+@beginTable
+@hdr3col{property, type, description}
+@row3col{virtualwidth, @ref overview_xrcformat_type_dimension,
+    Optional width passed to wxPropertyGrid::SetVirtualWidth().}
+@endTable
+
+A wxPropertyGrid may have @ref xrc_wxpropertygridproperty child objects.
+
+Example:
+@code
+<object class="wxPropertyGrid">
+    <property class="wxStringProperty">
+        <label>Name</label>
+        <value>Now And Then</value>
+    </property>
+    <property class="wxIntProperty">
+        <label>Year</label>
+        <value>2023</value>
+    </property>
+</object>
+@endcode
+
+Notice that wxPropertyGrid support in XRC is available in wxWidgets 3.3.0 and
+later only and you need to explicitly register its handler using
+@code
+    #include <wx/xrc/xh_propgrid.h>
+
+    AddHandler(new wxPropertyGridXmlHandler);
+@endcode
+to use it.
+
+
+@subsubsection xrc_wxpropertygridmanager wxPropertyGridManager
+
+@beginTable
+@hdr3col{property, type, description}
+@row3col{virtualwidth, @ref overview_xrcformat_type_dimension,
+    Optional width passed to wxPropertyGrid::SetVirtualWidth().}
+@endTable
+
+A wxPropertyGridManager contains one or more `page` elements each of which in
+turn contains one of more @ref xrc_wxpropertygridproperty child objects.
+
+Page elements may also have the following attributes:
+@beginTable
+@hdr3col{property, type, description}
+@row3col{label, @ref overview_xrcformat_type_text,
+    Label used for the page.}
+@row3col{columns, integer,
+    Optional column count passed to wxPropertyGridPageState::SetColumnCount().}
+@row3col{splitterpos, @ref overview_xrcformat_type_percent,
+    Optional splitter position passed to wxPropertyGridPageState::DoSetSplitter().
+    The `pos` argument of this function may be specified as `index` attribute
+    of this element and defaults to 0 if not given.}
+@row3col{choices, strings,
+    Space-separated list of double-quoted strings which can be used as choices
+    in a wxEnumProperty of this wxPropertyGridManager object by using the
+    `id` attribute of this element there.}
+@endTable
+
+Example:
+@code
+<object class="wxPropertyGridManager">
+    <page>
+        <property class="wxStringProperty">
+            <label>Name</label>
+            <value>Now And Then</value>
+        </property>
+        <property class="wxIntProperty">
+            <label>Year</label>
+            <value>2023</value>
+        </property>
+    </page>
+    <page>
+        <label>Reviews</label>
+        <choices id="stars">"1" "2" "3" "4" "5"</choices>
+        <property class="wxIntProperty">
+            <label>Metacritic</label>
+            <value>87</value>
+        </property>
+        <property class="wxEnumProperty">
+            <label>The Guardian</label>
+            <value>4</value>
+            <choices>@stars</choices>
+        </property>
+    </page>
+</object>
+@endcode
+
+Notice that wxPropertyGrid support in XRC is available in wxWidgets 3.3.0 and
+later only and you need to explicitly register its handler using
+@code
+    #include <wx/xrc/xh_propgrid.h>
+
+    AddHandler(new wxPropertyGridXmlHandler);
+@endcode
+to use it.
+
+
+@subsubsection xrc_wxpropertygridproperty wxPropertyGridProperty
+
+@beginTable
+@hdr3col{property, type, description}
+@row3col{label, @ref overview_xrcformat_type_text,
+    Property label appearing in the property grid.}
+@row3col{value, @ref overview_xrcformat_type_string,
+    Initial property value.}
+@row3col{flags, @ref overview_xrcformat_type_style,
+    Combination of wxPGPropertyFlags values without the leading `wxPG_PROP_`
+    prefix. Only "COLLAPSED", "DISABLED", "HIDDEN" and "NOEDITOR" are currently
+    allowed. (default: 0).}
+@row3col{tip, @ref overview_xrcformat_type_text,
+    Optional help string.}
+@row3col{expanded, @ref overview_xrcformat_type_bool,
+    For a property with children, may be used to specify whether it should be
+    expanded by default.}
+@row3col{choices, @ref overview_xrcformat_type_string,
+    Space-separated string containing the possible choices for the properties
+    using them, e.g. wxFlagsProperty or wxEnumProperty.}
+@endTable
+
+These elements define individual rows of @ref xrc_wxpropertygrid or @ref
+xrc_wxpropertygridmanager. Notice that they may be nested: a property with the
+class "wxPropertyCategory" will normally contain other properties inside it.
+
+
 @subsubsection xrc_wxpropertysheetdialog wxPropertySheetDialog
 
 @beginTable