pack-protocol: document "ERR" line

Since a807328 (connect.c: add a way for git-daemon to pass an error
back to client), git client recognizes "ERR" line and prints a
friendly message to user if an error happens at server side.

Document this.

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

Documentation/technical/pack-protocol.txt

@@ -60,6 +60,13 @@ process on the server side over the Git protocol is this:
      "0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" |
      nc -v example.com 9418
 
+If the server refuses the request for some reasons, it could abort
+gracefully with an error message.
+
+----
+  error-line     =  PKT-LINE("ERR" SP explanation-text)
+----
+
 
 SSH Transport
 -------------