Fix double negatives used with 'neither' in docs and comments

In many cases it should be 'either', and 'nor' should be 'or'
accordingly.

No changes to actual code.

Closes #22723.

docs/doxygen/mainpages/const_cpp.h

@@ -381,7 +381,7 @@ more details.
         if this macro is defined, the compilation options
         don't include compiler flags needed for multithreaded code generation. This
         implies that wxUSE_THREADS is 0 and also that other (non-wx-based) threading
-        packages cannot be used neither.}
+        packages cannot be used either.}
 @itemdef{wxNO_UNSAFE_WXSTRING_CONV,
         this symbol is not defined by wxWidgets itself, but can be defined by
         the applications using the library to disable unsafe implicit

docs/doxygen/overviews/changes_since28.h

@@ -35,7 +35,7 @@ of the information which was correct before doesn't apply any longer.
 For example, the notorious (due to the confusion they created) macros @c wxT()
 and @c _T() are not needed at all any longer. Basically, you can remove them
 from any code which used them. On the other hand, there is no particular harm
-in leaving them neither as the code will still compile and work correctly --
+in leaving them either as the code will still compile and work correctly --
 you only need to remove them if you think that your code looks tidier without
 them. You also don't need to use @c wxChar any longer but can directly use the
 standard @c wchar_t type even if, again, @c wxChar continues to work.

docs/doxygen/overviews/xrc_format.h

@@ -2038,9 +2038,9 @@ HTML markup. Note that the markup has to be escaped:
 @subsubsection xrc_wxsimplebook wxSimplebook
 
 wxSimplebook is similar to @ref xrc_wxnotebook "wxNotebook" but simpler: as it
-doesn't show any page headers, it doesn't use neither image list nor individual
+doesn't show any page headers, it uses neither image list nor individual
 page bitmaps and while it still accepts page labels, they are optional as they
-are not shown to the user neither.
+are not shown to the user either.
 
 So @c simplebookpage child elements, that must occur inside this object, only
 have the following properties: