(maint) Clarify docs for certificate_status

Prior to this commit, the SAVE and DELETE sections of the
certificate_status HTTP API docs were conflicting.
This commit corrects the documentation of the SAVE section
to match the actual Puppet behavior and be consistent with
the DELETE section.
Merge pull request #2996 from nwolfe/maint/master/clarify-cert-status-docs

(maint) Clarify docs for certificate_status