doc/gitattributes: mention non-recursive behavior

doc/gitattributes: mention non-recursive behavior

The gitattributes documentation claims that the pattern
rules are largely the same as for gitignore. However, the
rules for recursion are different.

In an ideal world, we would make them the same (if for
nothing else than consistency and simplicity), but that
would create backwards compatibility issues. For some
discussion, see this thread:

  https://public-inbox.org/git/slrnkldd3g.1l4.jan@majutsushi.net/

But let's at least document the differences instead of
actively misleading the user by claiming that they're the
same.

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