As the mobile version of search shows the description of item + the page title, we should always track the description of page. Even if the page doesn't explicitly access the description with Lua, the mobile version does.
The issue was raised by @Doc_James as there were cases of vandalism in description - and it is hard to track them.
Currently we use the whole entity to track changes, but once we make it less coarse we should make should we still track the description.
This could be done in a few ways:
* Provide extensions a hook to register additional usages which are applied to all pages (of a namespace). These would be taken into account in `AffectedPagesFinder`, but not not saved into `wbc_entity_usage`.
* Provide extensions a hook to register additional usages for specific pages. These would be saved in the `wbc_entity_usage` table and handled like regular usages.
* Provide some kind of API for this which registers usages as needed (similar to what is done in Lua). This might not be doable without rather nasty hacks.