Missing documentation concerning Bridge and PT configuration.

hackerencoder on IRC spotted these issues:

• The transports() method in BridgesConfig does not appear in our documentation: https://tpo.pages.torproject.net/core/doc/rust/arti_client/config/struct.BridgesConfig.html
• BridgeConfigBuilder's documentation doesn't explain itself very well if you don't already know the terminology (What is a direct connection? What is "k"? What is "v"? What is a setting?)

We fix these issues, then we should read over our generated documentation to make sure that it's clear to somebody who doesn't already know how to configure bridges and PTs.