(doc) Describe ca subcommands in puppet help

PUP-2801 requests ability to remove pending CSRs. The requester confirmed
that the puppet ca destroy command met that need. 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.
Merge branch 'pull-2783'

This closes GH-2783.