[css-fonts] updated wording for subscript/superscript glyph synthesis

I've pushed a minor edit to the CSS Fonts spec that adjusts the requirement
for user agents to use OS/2 subscript/superscript metrics for a given font
when synthesizing superscript/subscript glyphs.

When implementing font-variant-position in Firefox, I ran into the problem
that these font metrics are often defined oddly or incorrectly. So I've
relaxed the wording to simply require user agents to synthesize
subscript/superscript glyphs, along with a note desribing the problems
associated with the OS/2 metrics.

Regards,

John Daggett