scmutil: document that bookmarkrevs() ignores non-head bookmark branch

"- ancestors(head() and not bookmark(%s))" excludes the bookmarked branch
itself if bookmark(%s) is not a head. I'm a bit surprised by this behavior
while writing "log -B" tests, so let's document it.

mercurial/scmutil.py

@@ -2300,8 +2300,9 b' def _getrevsfromsymbols(repo, symbols):'
 
 
 def bookmarkrevs(repo, mark):
-    """
-    Select revisions reachable by a given bookmark
+    """Select revisions reachable by a given bookmark
+
+    If the bookmarked revision isn't a head, an empty set will be returned.
     """
     return repo.revs(
         b"ancestors(bookmark(%s)) - "