ACCUMULO-2539 Expand javadoc for Combiner to be more specific about combination implementation

Explicitly note that, most times, fetchColumnFamily or fetchColumns is desired and that columns
are only combined within a row. Only multiple versions for otherwise equal Keys are combined.
Merge branch '1.4.5-SNAPSHOT' into 1.5.2-SNAPSHOT

Conflicts:
	core/src/main/java/org/apache/accumulo/core/iterators/Combiner.java