docs/git: discuss server-side config for GIT_PROTOCOL

The v2 protocol requires that the GIT_PROTOCOL environment variable gets
passed around, but we don't have any documentation describing how this
is supposed to work. In particular, we need to note what server admins
might need to configure to make things work.

The definition of the GIT_PROTOCOL variable is probably the best place
for this, since:

  - we deal with multiple transports (ssh, http, etc).
    Transport-specific documentation (like the git-http-backend bits
    added in the previous commit) are helpful for those transports, but
    this gives a broader overview. Plus we do not have a specific
    transport endpoint program for ssh, so this is a reasonable place to
    mention it.

  - the server side of the protocol involves multiple programs. For now,
    upload-pack is the only endpoint which uses GIT_PROTOCOL, but that
    will likely expand in the future. We're better off with a central
    discussion of what the server admin might need to do. However, for
    discoverability, this patch adds a pointer from upload-pack's
    documentation.

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

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