try to document how to get rid of the backup warning

tsa/howto/postgresql.mdwn

@@ -462,8 +462,9 @@ backup server went down during that time.
  4. Either:
 
     * if a `.BASE.` backup is present after the missing files, it is
-      harmless insofar as the missing timeframe is not
-      necessary. TODO: how do we fix the warning anyways?
+      harmless to the extent that the missing timeframe is not
+      necessary. to remove the warnings, previous WAL files *do* need
+      to be removed, by hand.
 
     * if a `.BASE.` backup is *not* present after the missing files,
       the backup integrity is faulty, and a new base backup needs to