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