doc: clarify when row-level locks are released

They are released just like table-level locks.  Also clean up wording.
(Uses wording "rolled back to".)

Reported-by: me@sillymon.ch

Discussion: https://postgr.es/m/158074944048.1095.4309647363871637715@wrigleys.postgresql.org

Backpatch-through: 9.5

doc/src/sgml/mvcc.sgml

@@ -1039,7 +1039,7 @@ ERROR:  could not serialize access due to read/write dependencies among transact
      </tip>
 
    <para>
-    Once acquired, a lock is normally held till end of transaction.  But if a
+    Once acquired, a lock is normally held until the end of the transaction.  But if a
     lock is acquired after establishing a savepoint, the lock is released
     immediately if the savepoint is rolled back to.  This is consistent with
     the principle that <command>ROLLBACK</command> cancels all effects of the
@@ -1178,7 +1178,10 @@ ERROR:  could not serialize access due to read/write dependencies among transact
      conflicting locks on the same row, even in different subtransactions;
      but other than that, two transactions can never hold conflicting locks
      on the same row.  Row-level locks do not affect data querying; they
-     block only <emphasis>writers and lockers</emphasis> to the same row.
+     block only <emphasis>writers and lockers</emphasis> to the same
+     row.  Row-level locks are released at transaction end or during
+     savepoint rollback, just like table-level locks.
+
     </para>
 
      <variablelist>