Fix misleading documentation

Not every length parameter unit is accepted by Qt Style sheets.

Task-number: QTBUG-31907
Change-Id: I953c56caf68c1397ba24369f433d21c6f4772d0d
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

src/widgets/doc/src/widgets-and-layouts/stylesheet.qdoc

@@ -2901,6 +2901,9 @@
            \li \c ex: the ex width of the font (i.e., the height of 'x')
            \endlist
 
+           However, Qt is limited to font sizes in \c pt and \c px and any other
+           size must be in \c px, \c em or \c ex.
+
     \row
         \li \b{Number} \target Number
         \li A decimal integer or a real number