This is a subtask of rust-lang#32777.

It turns out that the subsequent lines of the error message comment
should be aligned like this.

The "turns the corresponding compiler warning" language may not be
strictly the most accurate—a lint check isn't the same as a compiler
warning; it emits a compiler warning if it's set to the `warn` level—
but it may be worth glossing over such distinctions in favor of simple,
familar phrasings for the sake of pedagogy; thanks to Guillaume Gomez
for the wording suggestion.

Let's also fix up the introductory clauses of the sentences about how to
fix the error to put a little more emphasis on the fact that the
`forbid` setting was probably there for a reason.