Say which regexp ranges should be avoided

* doc/lispref/searching.texi (Regexp Special): Say that
regular expressions like "[a-m-z]" and "[[:alpha:]-~]" should
be avoided, for the same reason that regular expressions like
"+" and "*" should be avoided: POSIX says their behavior is
undefined, and they are confusing anyway.  Also, explain
better what happens when the bound of a range is a raw 8-bit
byte; the old explanation appears to have been obsolete
anyway.  Finally, say that ranges like "[\u00FF-\xFF]" that
mix non-ASCII characters and raw 8-bit bytes should be
avoided, since it’s not clear what they should mean.
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