doc: clarify http.emptyAuth values
The existing description of http.emptyAuth explains the purpose of the setting but never says what values it accepts. Readers have to infer from context (or read the source) that it takes 'true', 'false', or 'auto', and what each one means. Document the three accepted values explicitly: * 'auto' (the default) only sends empty credentials when the server's 401 response advertises a mechanism that requires them, such as GSS-Negotiate. This matches the long-standing auto-detection behaviour added in 40a18fc77c (http: add an "auto" mode for http.emptyauth, 2017-02-25). * 'true' unconditionally sends empty credentials on the very first request, before any 401 response, for callers that know they want this behaviour up front. * 'false' disables the feature entirely; mechanisms that depend on empty credentials, such as GSS-Negotiate, will not work in this mode. Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>