(PUP-2801) Describe ca subcommands in puppet help

PUP-2801 requested ability to remove pending CSRs. The requester confirmed
that the puppet ca destroy command met that need. This suggests a need for
better documentation of the existing functionality.

This patch adds basic descriptions to puppet help ca for destroy, revoke, generate,
sign, print, fingerprint, and verify. The descriptions are largely sourced from the
puppet help cert descriptions of the same functionality.