Update Dagster CLI help descriptions

Summary:
Resolves https://github.com/dagster-io/dagster/issues/4239.

Adds aliases for help (-h/--help) and version (-v/--version). Also
adds missing descriptions for user facing Dagster CLI commands.

Test Plan: existing tests

Reviewers: alangenfeld, sashank

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D8195