cmd/cue: add documentation on the registry file format

[rogpeppe]

What version of CUE are you using (cue version)?

$ cue version
v0.8.0-alpha.3

Does this issue reproduce with the latest stable release?

N/A

What did you do?

cue help environment

What did you expect to see?

Documentation on the registry configuration file format

What did you see instead?

No such documentation.

The new file-based configuration is not documented at all yet. It's reasonably substantial, so maybe an extra help topic would be justified, to be pointed to by the cue help environment text.