gitattributes: document how external diff drivers relate to diff features

The "Defining an external diff driver" section explains how to
configure diff.<driver>.command but not how the driver relates to the
rest of Git's diff machinery.  In particular, the command only
replaces the textual patch: word diff, function context, color, and
the like cannot apply to its output, while the summary formats, blame,
and git log -L do not run it at all and keep using the builtin diff.

Spell this out so the scope of an external diff driver is clear.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.adoc

@@ -784,6 +784,17 @@ with the above configuration, i.e. `j-c-diff`, with 7
 parameters, just like `GIT_EXTERNAL_DIFF` program is called.
 See linkgit:git[1] for details.
 
+An external diff driver replaces the patch Git would otherwise
+produce for the path: Git runs the command and shows its output in
+place of its own.  Output features that post-process Git's diff do
+not apply to the driver's output; word diff, function context (`-W`),
+`--color-moved`, and coloring all act on Git's builtin diff, not the
+driver's output.
+The driver is consulted only when Git generates a textual patch.  The
+summary formats (`--stat`, `--numstat`, `--shortstat`, and
+`--dirstat`), `git blame`, and `git log -L` do not run it and
+continue to use Git's builtin diff.
+
 If the program is able to ignore certain changes (similar to
 `git diff --ignore-space-change`), then also set the option
 `trustExitCode` to true.  It is then expected to return exit code 1 if