Document which `errno` names are available on which builds

[dhalbert]

See #9733 for background. Right now errno is in py/, so it doesn't have inline documentation like shared-bindings.

[ricknun]

Thank you Dan. You are the most responsive person I've encountered regarding Adafruit issues. I finally quit submitting Adafruit-related MakeCode issues (https://github.com/microsoft/pxt-adafruit/issues) due to no apparent management there for years.

[dhalbert]

@ricknun. Thank you -- we all try to be responsive. We don't control MakeCode at all except to provide links to it: that code is entirely maintained by Microsoft, and they have varying amounts of time to spend.