Follow style guide for help strings

Improve help strings to follow the style guide:
* Capitalize first word of each help string
* Finish help strings with "."
* Improve wording and follow usual capitalization

Change-Id: Iecab34edd35175dcf1cccd4676e5cd91752549ad
Do substitution on overrides and defaults too

Some of our consuming projects are setting defaults or overrides
on config objects that look like $state_path/some/file and we're
no longer handling that correctly since commit
2422d4118c97734067ea0b37ae159bc2e3c492c5

Change-Id: I4324bda6f3de3cdf05f8eb3cf16052c04018dec8
Closes-Bug: #1282250
Merge "Follow style guide for help strings"
Merge "Do substitution on overrides and defaults too"