Various minor improvements to the xsprintf functions

Summary:

• Renamed Phutil*sprintfTestCase classes to use proper camel case.
• Added some type hints for function parameters, where appropriate.
• Use more specific (SPL) Exception types, where appropriate.
• Change ## to \` in comments.
• Wrap functions in @{function:...} when they appear in comments.

Test Plan: arc unit still works.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9788