docs/config: mention protocol implications of url.insteadOf

docs/config: mention protocol implications of url.insteadOf

If a URL rewrite switches the protocol to something
nonstandard (like "persistent-https" for "https"), the user
may be bitten by the fact that the default protocol
restrictions are different between the two. Let's drop a
note in insteadOf that points the user in the right
direction.

It would be nice if we could make this work out of the box,
but we can't without knowing the security implications of
the user's rewrite. Only the documentation for a particular
remote helper can advise one way or the other. Since we do
include the persistent-https helper in contrib/ (and since
it was the helper in the real-world case that inspired that
patch), let's also drop a note there.

Suggested-by: Elliott Cable <me@ell.io>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>