Update comments about MergeOperator::AllowSingleOperand

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

include/rocksdb/merge_operator.h

@@ -185,9 +185,9 @@ class MergeOperator {
 
   // Determines whether the MergeOperator can be called with just a single
   // merge operand.
-  // Override and return true for allowing a single operand. FullMergeV2 and
-  // PartialMerge/PartialMergeMulti should be implemented accordingly to handle
-  // a single operand.
+  // Override and return true for allowing a single operand. Both FullMergeV2
+  // and PartialMerge/PartialMergeMulti should be overridden and implemented
+  // correctly to handle a single operand.
   virtual bool AllowSingleOperand() const { return false; }
 
   // Allows to control when to invoke a full merge during Get.