screen-share: Document that --no-config option should be passed

With commit 'screen-share: Add option to start screen sharing on weston
star' an section option has been added to start screen-sharing by
default on all outputs. This has the side-effect of attempting to start
screen-share on the same (RDP) output performing the screen-share. That
happens due to re-loading the screen-share module once more.

This patch recommends users to use --no-config option or alternatively,
use a different configuration file to avoid that.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>