Fixing and adding some comments

Summary:
`MemTableList::Add()` neglected to mention that it took ownership of the reference held by its caller.

The comment in `MemTable::Get()` was wrong in describing the format of the key.

Test Plan: None

Reviewers: dhruba, sheki, emayanke, vamsi

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D7755

db/memtable.cc

@@ -116,7 +116,7 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s) {
   if (iter.Valid()) {
     // entry format is:
     //    klength  varint32
-    //    userkey  char[klength]
+    //    userkey  char[klength-8]
     //    tag      uint64
     //    vlength  varint32
     //    value    char[vlength]

db/memtable.h

@@ -49,7 +49,7 @@ class MemTable {
   // The caller must ensure that the underlying MemTable remains live
   // while the returned iterator is live.  The keys returned by this
   // iterator are internal keys encoded by AppendInternalKey in the
-  // db/format.{h,cc} module.
+  // db/dbformat.{h,cc} module.
   Iterator* NewIterator();
 
   // Add an entry into memtable that maps key to value at the

db/memtablelist.h

@@ -63,6 +63,7 @@ class MemTableList {
                       std::set<uint64_t>& pending_outputs);
 
   // New memtables are inserted at the front of the list.
+  // Takes ownership of the referenced held on *m by the caller of Add().
   void Add(MemTable* m);
 
   // Returns an estimate of the number of bytes of data in use.