Update WRITEMAP doc

libraries/liblmdb/lmdb.h

@@ -529,9 +529,11 @@ int  mdb_env_create(MDB_env **env);
 	 *		allowed. LMDB will still modify the lock file - except on read-only
 	 *		filesystems, where LMDB does not use locks.
 	 *	<li>#MDB_WRITEMAP
-	 *		Use a writeable memory map unless MDB_RDONLY is set. This is faster
-	 *		and uses fewer mallocs, but loses protection from application bugs
+	 *		Use a writeable memory map unless MDB_RDONLY is set. This uses
+	 *		fewer mallocs but loses protection from application bugs
 	 *		like wild pointer writes and other bad updates into the database.
+	 *		This may be slightly faster for DBs that fit entirely in RAM, but
+	 *		is slower for DBs larger than RAM.
 	 *		Incompatible with nested transactions.
 	 *		Do not mix processes with and without MDB_WRITEMAP on the same
 	 *		environment.  This can defeat durability (#mdb_env_sync etc).