DOCSP-14409 include units for db.collection.storageSize() return value

Author: ajhuh-mdb

source/reference/method/db.collection.storageSize.txt

@@ -12,8 +12,8 @@ db.collection.storageSize()
 
 .. method:: db.collection.storageSize()
 
-   :returns: The total amount of storage allocated to this collection
-             for document storage.
+   :returns: The total amount of storage in bytes allocated to this 
+             collection for document storage.
 
              If collection data is compressed (which is the
              :option:`default for WiredTiger <mongod