doc: Fix a few Example docs for PHP classes

doc: Fix a few Example docs for PHP classes

* "@example" is specifically for creating entries on the "Examples"
  page produced by Doxygen, which is not specific to individual classes
  or methods, but rather is meant to be about the software as a whole.
  There are currently four entries produced there, and they are all broken:
  - .../src/includes/rcfeed/RedisPubSubFeedEngine.php (unnamed example, broken)
  - .../src/tests/phpunit/MediaWikiTestCase.php (unnamed example)
  - 42 (empty page named "42")
  - user (empty page named "user")

* Remove usage and replace with either a regular paragraph that is
  part of the member/method/class description, or for code examples,
  use "@par" to create a sub heading, and then a @code-@endcode block.

Change-Id: I52c937fef4b377173f6c4dbad9dc4a9f8beae4ee