Update docs around positional arguments.
Add callout for subcommands.

Document that subcommand parsers are all or nothing.
Edits to collections docs
Merge pull request #1330 from bcrowe/3.0-collections

3.0 - Edits to collections docs
small fixes in a sentence for clarity
Merge pull request #1329 from cakephp/3.0-shell-args

3.0 shell args