Provide javadoc strings for all significant public symbols in o.a.h.h.client.*

While playing with the jshell via HBASE-25841, I noticed that several of our classes and methods in this package don't have javadoc. We should do this anyway, but with jshell's tab-completion to show javadoc, this became particularly noticeable. Specifically, the very first class one must use, ConnectionFactory, is without a class-level javadoc. I assume there are plenty more.