Add Javadoc comments to WikidataCache.

The methods in  WikidataCache are quite complicated, and it might not be
outwardly clear which ones are best to use for any particular purpose.

This patch adds Javadoc comments to the class so that it's a bit clearer how
to use it.

Change-Id: I2d0595c99a42592bde68114f1e6db2cb14686255