config: document value 2 for protocol.version

Update the config documentation to note the value `2` as an acceptable
value for the protocol.version config.

Signed-off-by: Brandon Williams <[email protected]>
Signed-off-by: Josh Steadmon <[email protected]>
Reviewed-by: Jonathan Nieder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bmwill

Documentation/config.txt

@@ -2587,6 +2587,8 @@ protocol.version::
 * `1` - the original wire protocol with the addition of a version string
   in the initial response from the server.
 
+* `2` - link:technical/protocol-v2.html[wire protocol version 2].
+
 --
 
 pull.ff::