doc/gitattributes: clarify location of config text

doc/gitattributes: clarify location of config text

The gitattributes documentation has a section on the "diff"
attribute, with subsections for each of the things you might
want to configure in your diff config section (external
diff, hunk headers, etc). The first such subsection
specifically notes that the definition of the diff driver
should go into $GIT_DIR/config, but subsequent sections do
not.

This location is implied if you are reading the
documentation sequentially, but it is not uncommon for a new
user to jump to (or be referred to) a specific section. For
a new user who does not know git well enough to recognize
the config syntax, it is not clear that those directives
don't also go into the gitattributes file.

This patch just mentions the config file in each subsection,
similar to the way it is mentioned in the first.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>