removing uneeded CSS declaration, adding a note, and reordering some declarations

originally submitted here:
https://github.com/richardmtl/_s/commit/5d15eef59b838200cd255bea6d954064
e87cf3cf ; additional note and explanation found there
Merge pull request #365 from richardmtl/master

_s: Remove unneeded CSS declaration, and document vendor prefixes.
Merge branch 'master' of git://github.com/Automattic/_s into upstream