make doctests fail on any warnings

[cosmicexplorer]

This is a useful setting to have on. We only have one doctest right now (for ProtocolAddress::new()), but this is useful when adding documentation in the constituent PRs of #467.

[jrose-signal]

I'm leery of this for the same reason we don't use deny(warnings): it makes it harder to use different compiler releases (even if this one doesn't affect downstream clients the same way). Is there a way to set this from the command line instead, so we can put it in our CI config but leave local builds alone?

[cosmicexplorer]

I totally hadn't considered how similar this is to deny(warnings) in general, and it's not clear how rustdoc can be made to read from cargo config, since #[doc] attributes do not seem to obviously have a way to be set except in the source code itself (looking at https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html#showing-warnings-in-doctests). I think I'll close this for now as I have had a lot of difficulty trying to figure this out and there doesn't seem to be a lot of benefit from it.