New BlockingObservable-specific diagram legend

A new diagram legend spells out the BlockingObservable-specific marble
diagram iconography.
Add javadoc comments to individual OperationFoo files
defer gets a diagram & javadoc + lots of javadoc changes in Observable

OperationDefer now has a javadoc explanation & marble diagram; I also
made a number of changes to the Observable javadocs
javadoc interlinkings and other javadoc improvements
diagram & description of combineLatest() operator