Follows-up Iaa1b4683c5c856.
* Match $IP pattern verbatim from most other WMF extensions.
* Improve descriptions a bit, and move/merge any meaningful
information from file docblock into class docblock. The file blocks
are visually ignored and identical in each file, and often out of
date or duplicated when given text separately from the class block.
See also similar changes in core:
https://gerrit.wikimedia.org/r/q/message:ingroup+owner:Krinkle
* Use `@internal` instead of `@private` as per Stable interface
policy.
Change-Id: I8bed9a625af003446c7e25f6b794931164767b5a
