branchmap: improve doc about BranchMapCache class

It was confusing that we are having two different branchcache and
BranchMapCache classes. The doc in BranchMapCache class was not much helpful
to understand the difference. This patch improves the doc there.

Differential Revision: https://phab.mercurial-scm.org/D5933

mercurial/branchmap.py

@@ -45,7 +45,7 b" subsettable = {None: 'visible',"
 
 
 class BranchMapCache(object):
-    """Cache mapping"""
+    """mapping of filtered views of repo with their branchcache"""
     def __init__(self):
         self._per_filter = {}