Only impose minimal height for buttons with non-empty label in wxMSW.

wxBU_EXACTFIT in wxMSW only affected the width of the button but not its
height, which was still made as big as the standard button size as otherwise
the button text was rendered badly. However there is no reason to do this for
the buttons without any text, so let wxBU_EXACTFIT be really exact, in both
directions, in this case.

Also document this rather non obvious wxBU_EXACTFIT behaviour.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73461 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/button.h

@@ -72,8 +72,13 @@
     @style{wxBU_BOTTOM}
         Aligns the label to the bottom of the button. Windows and GTK+ only.
     @style{wxBU_EXACTFIT}
-        Creates the button as small as possible instead of making it of the
-        standard size (which is the default behaviour ).
+        By default, all buttons are made of at least the standard button size,
+        even if their contents is small enough to fit into a smaller size. This
+        is done for consistency as most platforms use buttons of the same size
+        in the native dialogs, but can be overridden by specifying this flag.
+        If it is given, the button will be made just big enough for its
+        contents. Notice that under MSW the button will still have at least the
+        standard height, even with this style, if it has a non-empty label.
     @style{wxBU_NOTEXT}
         Disables the display of the text label in the button even if it has one
         or its id is one of the standard stock ids with an associated label: