document the diff driver textconv feature

document the diff driver textconv feature

This patch also changes the term "custom diff driver" to
"external diff driver"; now that there are more facets of a
"custom driver" than just external diffing, it makes sense
to refer to the configuration of "diff.foo.*" as the "foo
diff driver", with "diff.foo.command" as the "external
driver for foo".

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