Document name parsing for A/ and I/ objects

[mkelley]

Update the documentation to explain results for A/ and I/ objects. Despite being in a cometary orbit, A/ indicates it lacks cometary activity, so A/ objects parse as an asteroidal designation.

I/ objects may be cometary (2I) or asteroidal (1I), so they fail both parse_comet and parse_asteroid, since the target nature is unclear from the name.

[mkelley]

Addressed by #337