2.2.2 release preparations

[akarnokd]

Version 2.2.2 - September 6, 2018 (Maven)

Bugfixes

• Pull 6187: Fix refCount termination-reconnect race.

Documentation changes

• Pull 6171: Add explanation text to Undeliverable & OnErrorNotImplemented exceptions.
• Pull 6174: Auto-clean up RxJavaPlugins JavaDocs HTML.
• Pull 6175: Explain null observer/subscriber return errors from RxJavaPlugins in detail.
• Pull 6180: Update Additional-Reading.md.
• Pull 6180: Fix Flowable.reduce(BiFunction) JavaDoc; the operator does not signal NoSuchElementException.
• Pull 6193: Add "error handling" java docs section to fromCallable & co.
• Pull 6199: Fix terminology of cancel/dispose in the JavaDocs.
• Pull 6200: Fix toFuture marbles and descriptions.

[akarnokd]

Closing via https://github.com/ReactiveX/RxJava/releases/tag/v2.2.2