Document existing query tags

In #3295564: Views caching is broken with facets 2.0.4 we added the alter_cache_metadata query tag to search queries “fake executed” when saving a view. However, as far as I’m aware there currently isn’t any central place where all existing query tags are listed. For most, this is not a big problem, as you’ll likely encounter a particular search you want to alter and can just look at its tags to (potentially) find one that is specific enough for you.
However, in this case, the tag should in most cases be used when a query is generally altered, to do something different (or nothing at all) when saving a view. People cannot really do that if they have no way of knowing that this tag even exists.