docs: Remove shortname references.

In addition, align the removal notice next to 'key' with other
such notices (options.txt).
runtime: Cleanup refs to removed options

- {no,inv,}shortname
- {no,inv,}sn
- cm, key (remnants of cryptmethod)
Merge pull request #1623 from Pyrohh/cleanup-shortname

Docs: Remove 'shortname' references