MDB_RESERVE doc

Add mdb_put text to mdb_cursor_put description for people who
fail to draw logical conclusions.

libraries/liblmdb/lmdb.h

@@ -1458,7 +1458,8 @@ int  mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
 	 *		the database supports duplicates (#MDB_DUPSORT).
 	 *	<li>#MDB_RESERVE - reserve space for data of the given size, but
 	 *		don't copy the given data. Instead, return a pointer to the
-	 *		reserved space, which the caller can fill in later. This saves
+	 *		reserved space, which the caller can fill in later - before
+	 *		the next update operation or the transaction ends. This saves
 	 *		an extra memcpy if the data is being generated later. This flag
 	 *		must not be specified if the database was opened with #MDB_DUPSORT.
 	 *	<li>#MDB_APPEND - append the given key/data pair to the end of the