LinkCache: Minor clean up of documentation and variable names

LinkCache: Minor clean up of documentation and variable names

In preparation for conversion from MapCacheLRU to HashBagOStuff.

* Use early-return pattern for error and rejection cases.
* Add 'int' to @return for getGoodLinkFieldObj() per the
  values set in addGoodLinkObj().
* Add type hints for Title where possible.
* Avoid single-letter variables like $f and $s.
* Add visibility for public static methods.

Change-Id: Iab90539922f648c21cb147f749943a64f598e8de