Return non-zero height from GetMultiLineTextExtent("") in wxMSW

This restores the previous behaviour inadvertently changed by bfeae1922d
(Minor optimizations in GetMultiLineTextExtent(), 2020-06-10) and makes
it official by documenting it and adding tests checking for it.

It wasn't completely obviously if this was intentional or accidental
before, but at least wxStaticText itself relied on the old behaviour,
and chances are that so did some code outside the library, so make this
part of the API now.

See #18825.

interface/wx/dc.h

@@ -892,6 +892,12 @@ public:
         used for the text extent calculation, otherwise the currently selected
         font is used.
 
+        If @a string is empty, its horizontal extent is 0 but, for convenience
+        when using this function for allocating enough space for a possibly
+        multi-line string, its vertical extent is the same as the height of an
+        empty line of text. Please note that this behaviour differs from that
+        of GetTextExtent().
+
         @note This function works with both single-line and multi-line strings.
 
         @beginWxPerlOnly