Fix buffer() documentation to correctly describe onError behaviour (#5895)

* Corrected documentation for buffer onError behaviour

The operator does not emit the buffer in case of error. Suspect these
docs were copied over to 2.x before the fix from #3561 was merged.

* Corrected documentation for Flowable.buffer onError behaviour

* Update boundary buffer documentation to change source -> supplied

The word 'source' is incorrect here, since the buffer is emitted
whenever the supplied Publisher emits an item

Author: adamsp