resourceloader: Improve titleInfo docs and simplify title key

resourceloader: Improve titleInfo docs and simplify title key

* Document the structure of the in-process $titleInfo cache.
  Specifically, specify that it is not the value from getTitleInfo(),
  but rather a container for zero or more versions of such values.
  The reason this is fragmented is because ResourceLoaderContext
  is a parameter to most methods and as such, makes everything
  variable. Tracked as T99107.

* Make various bits easier to understand by consistently refering
  to the container keys as "batchKey", and referring to the internal
  keys as "titleKey".

* Centralise title key logic by moving to private method.

* Replace the internal creation of titleKey to be based on LinkTarget
  with plain namespace IDs and db keys, instead of invoking the
  expensive getPrefixedTitle function which involves quite a lot
  of overhead (TitleCodec, GenderCache, Database, Language,
  LocalisationCache, ..).

Change-Id: I701e5156ef7815a0e36caefae5871524eff3f688