Improve jails module documentation and names

Summary:
The jails.get_jail method is now jails.get.

Proofread module documentation.

Fixes T1278.

Test Plan: salt-call --local jails.get mumble

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1278

Differential Revision: https://devcentral.nasqueron.org/D1480