revset: mark `obsolete()` experimental

I assume not marking it was unintentional, since the obsolete concept it still
experimental.

Differential Revision: https://phab.mercurial-scm.org/D8405

mercurial/revset.py

@@ -1702,7 +1702,7 b' def none(repo, subset, x):'
 
 @predicate(b'obsolete()', safe=True)
 def obsolete(repo, subset, x):
-    """Mutable changeset with a newer version."""
+    """Mutable changeset with a newer version. (EXPERIMENTAL)"""
     # i18n: "obsolete" is a keyword
     getargs(x, 0, 0, _(b"obsolete takes no arguments"))
     obsoletes = obsmod.getrevs(repo, b'obsolete')