2.x: improve documentation of the default consumer types

[akarnokd]

Describe their single-use nature. Explicitly mention onStart. Explicitly mention dispose or cancel. Maybe add code examples.

• DisposableSubscriber
• DisposableCompletableObserver
• DisposableMaybeObserver
• DisposableSingleObserver
• DisposableObserver
• ResourceSubscriber (also remove mention of AsyncObserver).
• ResourceCompletableObserver
• ResourceMaybeObserver
• ResourceSingleObserver
• ResourceObserver

[akarnokd]

See #5210.

[akarnokd]

Closing via #5210