Improve documentation of POSIX extensions to strftime()

Mention that wxWidgets internal implementation may be used in this case
and replace the mention of MSVC (whose CRT now supports these
extensions) with MinGW.

interface/wx/datetime.h

@@ -925,7 +925,9 @@ public:
 
         Notice that POSIX @c "%g", @c "%G", @c "%V" and @c "%z" format
         specifiers are supported even if the standard library doesn't support
-        them (e.g. MSVC).
+        them (as is the case when using MinGW, for example), but wxWidgets own
+        implementation is used if any of them are used instead of calling
+        `strftime()`.
 
         It also accepts a few wxWidgets-specific extensions: you can optionally
         specify the width of the field to follow using @c printf(3)-like syntax