Improve usability/documentation for using API to connect to onion services

See the forum thread at https://forum.torproject.org/t/cannot-connect-to-onion-service/14917/2 :

Is there anything we can do in order to make this kind of code easier to write?

Some possible ideas include:

• Giving AutoBool a From<bool> implementation, and changing StreamPrefs::connect_to_onion_services to take an impl Into<AutoBool>.
• Making StreamPrefs (or the configuration) explicitly default to connecting to onion services if the the feature is enabled. (Don't we do that already?)
• Improving the documentation to say which of these steps are still necessary.
• Possibly, adding a section to doc/TROUBLESHOOTING.md about this.
• Giving a more useful error message, when trying to connect to an onion service is disabled or unsupported.