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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