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.
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif