Reconsider and document IntoTorAddr interface

On Reddit, a user asks:

Very minor, but why have a trait IntoTorAddr instead of TryInto (and/or TryFrom)? It feels unintuitive to me because normally Into denotes a conversion that always succeeds, and also because it's not apparent to me why it doesn't use one of the std traits.

I should document the rationale for this, if there is one. Otherwise I should change it.