Clarify what tx and rx mean in docs

[mrmonday]

This question comes up relatively frequently on IRC - "what do tx and rx mean?". This change adds a short explanation.

[alexcrichton]

If I were reading the docs I would then ask the question "Why isn't the sending type a Transmitter?" It may be useful to expand the explanation a little bit to something like:

// Create a shared channel which can be sent along from many tasks
// where tx is the sending half (tx for transmission), and rx is the receiving
// half (rx for receiving).
let (tx, rx) = channel();

[mrmonday]

Updated with your wording.

[brson]

Thanks for the docs!