[PATCH] grep: clarify what `grep.patternType=default` means

Back in the days when the "return to the default matching behavior" part was written in 84befcd0 (grep: add a grep.patternType configuration setting, 2012-08-03), grep.extendedRegexp was the only way to configure the behaviour since b22520a3 (grep: allow -E and -n to be turned on by default via configuration, 2011-03-30).

It was understandable that we referred to the behaviour that honors the older configuration variable as "the default matching" behaviour. It is fairly clear in its log message:

    When grep.patternType is set to a value other than "default", the
    grep.extendedRegexp setting is ignored. The value of "default" restores
    the current default behavior, including the grep.extendedRegexp
    behavior.

But when the paragraph is read in isolation by a new person who is not aware of that backstory (which is the synonym for "most users"), the "default matching behaviour" can be read as "how 'git grep' behaves without any configuration variables or options", which is "match the pattern as BRE".

Clarify what the passage means by elaborating what the phrase "default matching behaviour" wanted to mean.

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

 * Whether the eventual deprecation of grep.extendedRegexp is a good
   idea, we'd need something like this to clarify what these two
   variables are meant to interact with each other first.

 Documentation/config/grep.txt | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)