upgrade: improve documentation of matchrevlog()

It was not clear what selected for cloning meant. I updated with an extra line
of description.

mercurial/upgrade.py

@@ -743,7 +743,10 b' def getsidedatacompanion(srcrepo, dstrep'
 
 
 def matchrevlog(revlogfilter, entry):
-    """check is a revlog is selected for cloning
+    """check if a revlog is selected for cloning.
+
+    In other words, are there any updates which need to be done on revlog
+    or it can be blindly copied.
 
     The store entry is checked against the passed filter"""
     if entry.endswith(b'00changelog.i'):