Improve javadoc descriptions to marker clustering algorithms

[wangela]

The marker clustering utility supports specifying custom clustering algorithms for various clustering scenarios. The current javadoc for algorithm types under com.google.maps.android.clustering.algo does not indicate which algorithm is best suited for which scenario.

We can reduce confusion by providing better guidance on which algorithm to select within the javadoc. For example, for clustering large numbers of markers (reported in #960 and #1199), we could modify the javadoc to read:

For managing large numbers of items (>1000 markers). This algorithm works the same way as {@link NonHierarchicalDistanceBasedAlgorithm} but works, only in visible area. It requires {@link shouldReclusterOnMapMovement} to be true in order to re-render clustering when camera movement changes the visible area.