Add missing channel field to process log event docs

Process log events (from both stdout and stderr) have channel
information in their payload. The docs have been updated to reflect
this.

Author: liamadamson

docs/events.rst

@@ -660,7 +660,7 @@ Body Description
 
 .. code-block:: text
 
-   processname:name groupname:name pid:pid
+   processname:name groupname:name pid:pid channel:stdout
    data
 
 ``PROCESS_LOG_STDERR`` Event Type
@@ -680,7 +680,7 @@ Body Description
 
 .. code-block:: text
 
-   processname:name groupname:name pid:pid
+   processname:name groupname:name pid:pid channel:stderr
    data
 
 ``PROCESS_COMMUNICATION`` Event Type