grep: clarify what `grep.patternType=default` means

We documented that with grep.patternType set to default, the "git
grep" command returns to "the default matching behavior" in 84befcd0
(grep: add a grep.patternType configuration setting, 2012-08-03).

The grep.extendedRegexp configuration variable was the only way to
configure the behavior before that, after b22520a3 (grep: allow -E
and -n to be turned on by default via configuration, 2011-03-30)
introduced it.

It is understandable that we referred to the behavior that honors
the older configuration variable as "the default matching"
behavior.  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 behavior" 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 behavior" wanted to mean.

Helped-by: Johannes Altmanninger <aclopte@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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