[css-fonts] Use CSS value definition syntax to define <family-name> and <generic-family>

In https://lists.w3.org/Archives/Public/www-style/2009Apr/0351.html Bert
Bos defined <family-name> and <generic-family> using CSS value definition
syntax:

<generic-family> = serif | sans-serif | cursive | fantasy | monospace
<family-name> = STRING | IDENT+

Though it seems this syntax was never picked up by the spec. Instead the
accepted values are currently only described in words.

So, while the syntax above may not be totally correct yet, I suggest to add
definitions in value definition syntax for those two values.

Sebastian