Fix errors in hot_spec doc

allowed_values is not written properly in an example, and fix a
sentence.

Change-Id: I6544033365b937e1e0fc33d23c143d6a3291273b
Improve help strings

Follow oslo.config style guide for help strings better to create
consistent help strings:
* Capitalize first word of each help string
* Finish help strings with "."
* Improve wording
* Add missing space between strings

Change-Id: Ia86734d44fdf4eb3711fef800fe6bdfd1a1b59d4
Merge "Fix errors in hot_spec doc"
Merge "Improve help strings"