WIP: Disrecommend the use of display socket name wayland-0

This is a documentation change which recommends that Wayland compositors in general stop providing a socket named wayland-0, thereby preventing likely-accidental connections from clients run in an unrelated context. It does not deprecate any functions or announce an intent to change behavior in the future. See the commit message for details.

Some history: There have been a few short discussions about avoiding (or changing) the fallback to wayland-0 on IRC in #wayland. There's a related issue at https://gitlab.gnome.org/GNOME/gtk/-/issues/1741, which may have led to Weston and Sway stopping to provide wayland-0 by default (weston!486 (merged), https://github.com/swaywm/sway/pull/5688). Going back further, there was a patch https://lists.freedesktop.org/archives/wayland-devel/2015-August/023909.html , which actually dropped the wayland-0 default, was committed as fb7e1302, but then reverted as c7dbaa1c because it broke a number of existing use cases, including the then-current version of gtk+.