3.x: Replace the verbose @return entires with simpler ones

[akarnokd]

Many operator's documentation repeats (more or less) its first sentence as part of @return. This adds unnecessary verbosity, opportunity for them to become desynced and is extra effort to maintain.

Instead, they should be as simple as @return the new {@code Type} instance.

• Flowable
• Observable
• Maybe
• Single
• Completable
• ParallelFlowable

\@return (?!the new)