Improve rtc documentation

[dhalbert]

There should be a simple example in the doc, and rtc.RTC datetime should say it takes a struct_time, etc.

[nickzoic]

Yeah, it is a bit obscure. There's sample code in PR #1534 ... the stuff to update is the \\| stuff in shared-bindings/rtc/__init__.c, right? I probably should have done that as part of #1534 already ...

[tannewt]

Yup, \\| lines are filtered out and treated as an rst file.