Dedicate user-, custom- & dev- prefixes

Allocating `user-*`, `custom-*` and `dev-*` feature prefixes to end
users.

resolves: QubesOS/qubes-issues#9557

doc/manpages/qvm-features.rst

@@ -341,6 +341,12 @@ Boot the HVM qube via UEFI boot, instead of legacy one. Support for this boot
 mode is experimental and may not work in all the cases. It is ignored for
 non-HVM qubes.
 
+Custom and user specific features
+---------------------------------
+
+Features with `x-` prefix is dedicated to end users and is guarantied to never
+be used for Qubes OS future internal features.
+
 Authors
 -------