Convert API module to use i18n help/documentation.

MediaWiki core change I04b1a384 added support for i18n of API module
help. This takes advantage of that while still maintaining backwards
compatibility with earlier versions of MediaWiki.

Once support for MediaWiki before 1.25 is dropped, the methods marked
deprecated in this patch may be removed.

Bug: T76900
Change-Id: Ib55a612258edc10972731b0504b9bae5dc230753