Improve documentation on duplex.rs

Adding example and short documentation for methods.
If I were unsure what `send_opt` did, this documentation doesn't really help me much because it just takes the 8-letter method name and expands it to a bit of english.

If we're adding documentation to these methods, then this should document the semantics of why it exists, what's optional about it, etc. This also applies to the documentation below as well.