Add a comment to the EnvFlags

Author: Kerollmops

heed/src/mdb/lmdb_flags.rs

@@ -28,6 +28,8 @@ bitflags! {
         /// Use asynchronous msync when MDB_WRITEMAP is used.
         const MAP_ASYNC = ffi::MDB_MAPASYNC;
         /// Tie reader locktable slots to MDB_txn objects instead of to threads.
+        // Note to self: When removing this flag from here, we must introduce an
+        //               internal-only AllEnvFlags akin to the AllDatabaseFlags bitflags.
         #[deprecated(since="0.22.0", note="please use `EnvOpenOptions::read_txn_with_tls` or `EnvOpenOptions::read_txn_without_tls` instead")]
         const NO_TLS = ffi::MDB_NOTLS;
         /// Don't do any locking, caller must manage their own locks.