delta-find: add a small docstring to deltacomputer

As we are about to introduce another object related to finding delta. So lets
have a minimal docstring to the existing one.

mercurial/revlogutils/deltas.py

@@ -1083,6 +1083,12 b' class SnapshotCache:'
 
 
 class deltacomputer:
+    """object capable of computing delta and finding delta for multiple revision
+
+    This object is meant to compute and find multiple delta applied to the same
+    revlog.
+    """
+
     def __init__(
         self,
         revlog,