Copy-edit doc strings and comments wrt bignum and fixnum

Copy-edit doc strings and comments wrt bignum and fixnum

Don't say that a value is a fixnum if small enough and a bignum
otherwise, because that is now how Lisp integers always work,
and we generally don't expose the fixnum/bignum difference to the user
unless really necessary.

* src/charset.c (Fencode_char):
* src/editfns.c (Fuser_uid, Fuser_real_uid, Fgroup_gid)
(Fgroup_real_gid, Femacs_pid):
* src/font.c (Ffont_variation_glyphs):
* src/process.c (Fprocess_id):
Edit doc strings.
* src/pgtkselect.c:
* src/xselect.c: Edit comments.