cmdserver: document message-encodings and channel output options

While writing the previous patch, I noticed these options are undocumented.
In my testing, a separate status/error message channel works well in GUI
frontend as we no longer have to sort out data and message from mixed outputs.
So let's mark it as not experimental.

mercurial/commandserver.py

@@ -191,7 +191,6 b' class channeledinput(object):'
 
 
 def _selectmessageencoder(ui):
-    # experimental config: cmdserver.message-encodings
     encnames = ui.configlist(b'cmdserver', b'message-encodings')
     for n in encnames:
         f = _messageencoders.get(n)
@@ -234,9 +233,6 b' class server(object):'
             self.ui = self.ui.copy()
             setuplogging(self.ui, repo=None, fp=self.cdebug)
 
-        # TODO: add this to help/config.txt when stabilized
-        # ``channel``
-        #   Use separate channel for structured output. (Command-server only)
         self.cmsg = None
         if ui.config(b'ui', b'message-output') == b'channel':
             encname, encfn = _selectmessageencoder(ui)

mercurial/configitems.py

@@ -204,7 +204,7 b' coreconfigitem('
     b'cmdserver', b'max-repo-cache', default=0, experimental=True,
 )
 coreconfigitem(
-    b'cmdserver', b'message-encodings', default=list, experimental=True,
+    b'cmdserver', b'message-encodings', default=list,
 )
 coreconfigitem(
     b'cmdserver',

mercurial/helptext/config.txt

@@ -413,6 +413,12 b' for credentials as usual if required by '
 
 Controls command server settings. (ADVANCED)
 
+``message-encodings``
+    List of encodings for the ``m`` (message) channel. The first encoding
+    supported by the server will be selected and advertised in the hello
+    message. This is useful only when ``ui.message-output`` is set to
+    ``channel``. Supported encodings are ``cbor``.
+
 ``shutdown-on-interrupt``
     If set to false, the server's main loop will continue running after
     SIGINT received. ``runcommand`` requests can still be interrupted by
@@ -2383,6 +2389,8 b' User interface controls.'
 ``message-output``
     Where to write status and error messages. (default: ``stdio``)
 
+    ``channel``
+      Use separate channel for structured output. (Command-server only)
     ``stderr``
       Everything to stderr.
     ``stdio``