docs(admin) clarify PUT endpoints behavior
docs(admin) clarify PUT endpoints behavior

As a follow-up to:

Kong/kong#3007 (PR)
Kong/kong#2774 (issue)

This commit adds clarifications as to the current behavior of PUT
endpoints on the Admin API. As much as we acknowledge that their
behavior is not compliant with the HTTP specifications, we do wish to
clarify their behavior to avoid confusion as of today, until a different
implementation sees the light of day.

Mashape/getkong.org,Mashape/getkong.org,Mashape/getkong.org