doc: mention that proxies must be completely transparent

doc: mention that proxies must be completely transparent

We already document in the FAQ that proxies must be completely
transparent and not modify the request or response in any way, but add
similar documentation to the http.proxy entry.  We know that while the
FAQ is very useful, users sometimes are less likely to read in favor of
the documentation specific to an option or command, so adding it in both
places will help users be adequately informed.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>