2.x: improve io.reactivex.schedulers.Schedulers documentation

[akarnokd]

• Better explain each scheduler type.
• Repeat system parameters for each scheduler type.
• Remove mentions of SchedulerLifecycle.
• Explain how from handles Executor, ExecutorService and ScheduledExecutorService.
• Explain getting back to the current thread requires a blocking scheduler.
• Mention the SchedulerSupport annotations.

[akarnokd]

See #5223.

[akarnokd]

Closing via #5223.