obsolete: clarify that 'successorssets' returns the latest successors

We do not return the first successors we found, we returns the latest (non
obsolete  (mostly)) one following the obsolete link transitively. We update the
documentation to make this clean.

mercurial/obsolete.py

@@ -841,7 +841,7 b' def foreground(repo, nodes):'
 
 
 def successorssets(repo, initialnode, cache=None):
-    """Return all set of successors of initial nodes
+    """Return set of all latest successors of initial nodes
 
     The successors set of a changeset A are the group of revisions that succeed
     A. It succeeds A as a consistent whole, each revision being only a partial