Minor cleanup of some session code

Summary: Ref T4398. Add some documentation and use phutil_units().

Test Plan:

• Established a web session.
• Established a conduit session.
• Entered and exited hisec.
• Used "Sessions" panel to examine results.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4398

Differential Revision: https://secure.phabricator.com/D8924