Standardize usage/command registration.

Standardize usage statements:
* Standardize on "Display" for all status commands
* Verb tense
* [] for optional arguments
* Capitalization
* Line feeds and other whitespace
* Options
* Punctuation
* Where to specify default scope

Add a top-level "cluster --help" usage.

Shortened index example in 'cluster partition' to fit in 80 chars.

Ignored text in 'cluster members' since that will be removed shortly.

Spurious whitespace cleanup.
Merge pull request #677 from basho/feature/jrd/normalize-usage

Standardize usage/command registration

Reviewed-by: andrewjstone