Update SPI javadoc for cache impls to note they are monotonically increasing counters

[DomGarguilo]

Looked at the cache impls in Accumulo, these counters are monotonically increasing counters for those. We should probably add that to the SPI javadoc.

Originally posted by @keith-turner in #4518 (comment)

This discussion was in relation to newly added metrics to track accumulo block caches.