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>

Documentation/git-config.txt

@@ -33,10 +33,13 @@ escaped.
 
 Multiple lines can be added to an option by using the `--add` option.
 If you want to update or unset an option which can occur on multiple
-lines, a POSIX regexp `value-pattern` needs to be given.  Only the
-existing values that match the regexp are updated or unset.  If
-you want to handle the lines that do *not* match the regex, just
-prepend a single exclamation mark in front (see also <<EXAMPLES>>).
+lines, a `value-pattern` (which is an extended regular expression,
+unless the `--fixed-value` option is given) needs to be given.  Only the
+existing values that match the pattern are updated or unset.  If
+you want to handle the lines that do *not* match the pattern, just
+prepend a single exclamation mark in front (see also <<EXAMPLES>>),
+but note that this only works when the `--fixed-value` option is not
+in use.
 
 The `--type=<type>` option instructs 'git config' to ensure that incoming and
 outgoing values are canonicalize-able under the given <type>.  If no