Make some WriteOptions defaults more explicit

Summary:
Some WriteOptions defaults were not clearly documented. So, added comments to make the defaults more explicit.
Closes https://github.com/facebook/rocksdb/pull/2984

Differential Revision: D6014500

Pulled By: sagar0

fbshipit-source-id: a28078818e335e42b303c1fc6fbfec692ed16c7c

include/rocksdb/options.h

@@ -1089,6 +1089,7 @@ struct WriteOptions {
 
   // If true, writes will not first go to the write ahead log,
   // and the write may got lost after a crash.
+  // Default: false
   bool disableWAL;
 
   // If true and if user is trying to write to column families that don't exist
@@ -1099,6 +1100,7 @@ struct WriteOptions {
 
   // If true and we need to wait or sleep for the write request, fails
   // immediately with Status::Incomplete().
+  // Default: false
   bool no_slowdown;
 
   // If true, this write request is of lower priority if compaction is