protocol-capabilities.txt: document symref

In 7171d8c15f ("upload-pack: send symbolic ref information as
capability"), we added a symref capability to the pack protocol, but it
was never documented. Adapt the patch notes from that commit and add
them to the capabilities documentation.

While we're at it, add a disclaimer to the top of
protocol-capabilities.txt noting that the doc only applies to v0/v1 of
the wire protocol.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/protocol-capabilities.txt

@@ -1,6 +1,10 @@
 Git Protocol Capabilities
 =========================
 
+NOTE: this document describes capabilities for versions 0 and 1 of the pack
+protocol. For version 2, please refer to the link:protocol-v2.html[protocol-v2]
+doc.
+
 Servers SHOULD support all capabilities defined in this document.
 
 On the very first line of the initial server response of either
@@ -172,6 +176,20 @@ agent strings are purely informative for statistics and debugging
 purposes, and MUST NOT be used to programmatically assume the presence
 or absence of particular features.
 
+symref
+------
+
+This parameterized capability is used to inform the receiver which symbolic ref
+points to which ref; for example, "symref=HEAD:refs/heads/master" tells the
+receiver that HEAD points to master. This capability can be repeated to
+represent multiple symrefs.
+
+Servers SHOULD include this capability for the HEAD symref if it is one of the
+refs being sent.
+
+Clients MAY use the parameters from this capability to select the proper initial
+branch when cloning a repository.
+
 shallow
 -------