Summary:
Updated comments around AllowSingleOperand.
Reason: A couple of users were confused and encountered issues due to no overriding PartialMerge with AllowSingleOperand=true.

I'll also look into modifying the default merge operator implementation so that overriding PartialMerge is not mandatory when AllowSingleOp=true.
Closes https://github.com/facebook/rocksdb/pull/3659

Differential Revision: D7422691

Pulled By: sagar0

fbshipit-source-id: 3d075a6ced0120f5d65cb7ae5412936f1862f342