Document the availability of OS::stack_name and OS::stack_id

Add documentation about the availablility of HOT pseudo parameters
and how to access them.

Implements: blueprint make-stack-name-available-in-template
Change-Id: Iee07aba5cf1b24321191f04282eb0c89a54382cb
Merge "Document the availability of OS::stack_name and OS::stack_id"