obsolete: small doc update for 'successorssets'

Clarify successorssets documentation before we start updating the main function.

This patch serie will introduce the successorssets template, the opposite of
predecessor templates.

Successors will use successorssets function and requires some improvement so
before doing that, we clean up successorssets a bit.

mercurial/obsutil.py

@@ -338,15 +338,16 b' def successorssets(repo, initialnode, ca'
     successors set. To distinguish this from a pruned changeset, the successor
     set will contain itself only, i.e. [(A,)].
 
-    Finally, successors unknown locally are considered to be pruned (obsoleted
-    without any successors).
+    Finally, final successors unknown locally are considered to be pruned
+    (pruned: obsoleted without any successors). (Final: successors not affected
+    by markers).
 
-    The optional `cache` parameter is a dictionary that may contain precomputed
-    successors sets. It is meant to reuse the computation of a previous call to
-    `successorssets` when multiple calls are made at the same time. The cache
-    dictionary is updated in place. The caller is responsible for its life
-    span. Code that makes multiple calls to `successorssets` *must* use this
-    cache mechanism or suffer terrible performance.
+    The optional `cache` parameter is a dictionary that may contains
+    precomputed successors sets. It is meant to reuse the computation of a
+    previous call to `successorssets` when multiple calls are made at the same
+    time. The cache dictionary is updated in place. The caller is responsible
+    for its life span. Code that makes multiple calls to `successorssets`
+    *should* use this cache mechanism or risk a performance hit.
     """
 
     succmarkers = repo.obsstore.successors