Doc: Update notes about libc collation versions.

Doc: Update notes about libc collation versions.

The per-index collation version tracking feature was reverted, but we
still have the ability to ask Windows (352f6f2d) and FreeBSD
(ca051d8b) for collation versions to store in pg_collation.collversion.
So, from the reverted patch, take a few words of documentation about
libc on all three supported OSes to replace the pre-existing note that
mentioned only glibc.

Discussion: http://postgr.es/m/CA%2BhUKGLhj5t1fcjqAu8iD9B3ixJtsTNqyCCD4V0aTO9kAKAjjA%40mail.gmail.com