Clarifying missed importance of attrs in hot-spec

Following the discussion in commit
I27f3a732480a851fd2eceb83e5fcc850ee59bb73, description
of some attributes (such as type in Resources section or
value in Outputs section) gives unclear answer, which
attribute is required and which is optional.

Change-Id: Ie6e207e427a577d46acb731cee495834faecd90d
Partial-bug: #1386242
Merge "Clarifying missed importance of attrs in hot-spec"