doc/http-backend: clarify "half-auth" repo configuration

When the http-backend is set up to allow anonymous read but authenticated write, the http-backend manual suggests catching only the "/git-receive-pack" POST of the packfile, not the initial "info/refs?service=git-receive-pack" GET in which we advertise refs. This does work and is secure, as we do not allow any write during the info/refs request, and the information in the ref advertisement is the same that you would get from a fetch. However, the configuration required by the server is slightly more complex. The default `http.receivepack` setting is to allow pushes if the webserver tells us that the user authenticated, and otherwise to return a 403 ("Forbidden"). That works fine if authentication is turned on completely; the initial request requires authentication, and http-backend realizes it is OK to do a push. But for this "half-auth" state, no authentication has occurred during the initial ref advertisement. The http-backend CGI therefore does not think that pushing should be enabled, and responds with a 403. The client cannot continue, even though the server would have allowed it to run if it had provided credentials. It would be much better if the server responded with a 401, asking for credentials during the initial contact. But git-http-backend does not know about the server's auth configuration (so a 401 would be confusing in the case of a true anonymous server). Unfortunately, configuring Apache to recognize the query string and apply the auth appropriately to receive-pack (but not upload-pack) initial requests is non-trivial. The site admin can work around this by just turning on http.receivepack explicitly in its repositories. Let's document this workaround. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>