git-config: better document default behavior for --include

As described in the commit message of 9b25a0b (config: add
include directive, 2012-02-06), the `--include` option is
only on by default in some cases. But our documentation
described it as just "defaults to on", which doesn't tell
the whole story.

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

Documentation/git-config.txt

@@ -181,7 +181,9 @@ See also <<FILES>>.
 --includes::
 --no-includes::
 	Respect `include.*` directives in config files when looking up
-	values. Defaults to on.
+	values. Defaults to `off` when a specific file is given (e.g.,
+	using `--file`, `--global`, etc) and `on` when searching all
+	config files.
 
 [[FILES]]
 FILES