exprIsHNF is actually checking if its argument is in WHNF

The exprIsHNF function will not descend into lambdas if they bind run-time variables:

    is_hnf_like (Lam b e)        = isRuntimeVar b || is_hnf_like e

So the documentation should probably reflect that it actually checks WHNF, because it currently emphasises that it checks head normal form:

-- | exprIsHNF returns true for expressions that are certainly /already/
-- evaluated to /head/ normal form.  This is used to decide whether it's ok
-- to change:

And perhaps we should even change the function name?