More regexp advice and clarifications

More regexp advice and clarifications

* doc/lispref/searching.texi (Regexp Special): Simplify style
advice for order of ], ^, and - in character alternatives.
Stick with saying that it’s not a good idea to put ‘-’ after a
range.  Remove the special case about raw 8-bit bytes and
unibyte characters, as this documentation is confusing and
seems to be incorrect in some cases.  Say that z-a is the
preferred style for reversed ranges, since it’s clearer and is
typically what’s used in practice.  Mention some bad styles:
duplicates in character alternatives, ranges that denote <=3
characters, and ‘-’ as the first character.