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

[Junio C Hamano <gitster@pobox.com>]

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(-)

diff --git a/Documentation/config/grep.txt b/Documentation/config/grep.txt
index 44abe45a7c..72f5e03614 100644
--- a/Documentation/config/grep.txt
+++ b/Documentation/config/grep.txt
@@ -8,7 +8,8 @@ grep.patternType::
 	Set the default matching behavior. Using a value of 'basic', 'extended',
 	'fixed', or 'perl' will enable the `--basic-regexp`, `--extended-regexp`,
 	`--fixed-strings`, or `--perl-regexp` option accordingly, while the
-	value 'default' will return to the default matching behavior.
+	value 'default' will use the settings of `grep.extendedRegexp` option
+	to choose between `basic` and `extended`.
 
 grep.extendedRegexp::
 	If set to true, enable `--extended-regexp` option by default. This
-- 
2.34.1-251-g6783e24198