Doc: improve description of regexp character classes.

Doc: improve description of regexp character classes.

Define the meanings of the POSIX-spec character classes in line,
rather than referring to the ctype(3) man page.  That man page
doesn't even exist on many modern systems, and if it does exist
it probably says the wrong things about non-ASCII characters.
Also document our non-POSIX-spec "ascii" character class.

Also, point out here that this behavior is controlled by collation or
LC_CTYPE, since the existing text explaining that is pretty far away.

Per gripe from Geert Lobbestael.  Given the lack of prior complaints,
I'm not excited about back-patching this.

Discussion: https://postgr.es/m/155837022049.1359.2948065118562813468@wrigleys.postgresql.org