config doc: value-pattern is not necessarily a regexp

The introductory part of the "git config --help" mentions the
optional value-pattern argument, but give no hint that it can be
something other than a regular expression (worse, it just says
"POSIX regexp", which usually means BRE but the regexp the command
takes is ERE).  Also, it needs to be documented that the '!' prefix
to negate the match, which is only mentioned in this part of the
document, works only with regexp and not with the --fixed-value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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