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.

tests/regex/regextest.cpp

@@ -168,7 +168,7 @@ bool RegExTestCase::parseFlags(const wxString& flags)
             // Also skip the (there is only one) test using POSIX-specified
             // handling of unmatched ')' as a non-special character -- PCRE
             // doesn't support this and it doesn't seem worth implementing
-            // support for this ourselves neither.
+            // support for this ourselves either.
             case 'U':
                 return false;