Lint should say when it is checking a rule

It is rather confusing that when lint finds an error in a rule attached
to a binder, it reports the error as in the RHS, not the rule:
  ...
  In the RHS of foo

We add a clarifying line:
  ...
  In the RHS of foo
  In a rule attached to foo

The implication that the rule lives inside the RHS is a bit odd, but
this niggle is already present for unfoldings, whose pattern we are
following.