document 'allow-tip-sha1-in-want' capability

See 390eb36 (upload-pack: optionally allow fetching from the tips of
hidden refs - 2013-01-28) for more information.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/protocol-capabilities.txt

@@ -210,3 +210,10 @@ be shown when processing the received pack. A send-pack client should
 respond with the 'quiet' capability to suppress server-side progress
 reporting if the local progress reporting is also being suppressed
 (e.g., via `push -q`, or if stderr does not go to a tty).
+
+allow-tip-sha1-in-want
+----------------------
+
+If the upload-pack server advertises this capability, fetch-pack may
+send "want" lines with SHA-1s that exist at the server but are not
+advertised by upload-pack.