doc: add detail explanation for 'present()' predicate of revsets stable

doc: add detail explanation for 'present()' predicate of revsets

--- a/mercurial/revset.py	Fri May 18 14:34:33 2012 -0500
+++ b/mercurial/revset.py	Wed May 16 17:02:30 2012 +0900
@@ -845,6 +845,10 @@
     """``present(set)``
     An empty set, if any revision in set isn't found; otherwise,
     all revisions in set.
+
+    If any of specified revisions is not present in the local repository,
+    the query is normally aborted. But this predicate allows the query
+    to continue even in such cases.
     """
     try:
         return getset(repo, subset, x)