Better comment for inplace_update_support

Summary: See https://github.com/facebook/rocksdb/issues/215

Test Plan: none

Reviewers: dhruba, sdong, ljin, yhchiang, nkg-

Reviewed By: nkg-

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20769

include/rocksdb/options.h

@@ -487,7 +487,9 @@ struct ColumnFamilyOptions {
       TablePropertiesCollectorFactories;
   TablePropertiesCollectorFactories table_properties_collector_factories;
 
-  // Allows thread-safe inplace updates.
+  // Allows thread-safe inplace updates. If this is true, there is no way to
+  // achieve point-in-time consistency using snapshot or iterator (assuming
+  // concurrent updates).
   // If inplace_callback function is not set,
   //   Put(key, new_value) will update inplace the existing_value iff
   //   * key exists in current memtable