Javadoc: Explain explicitly that the Emitter methods should be called non-concurrently with generate

[akarnokd]

The Javadocs of the generate methods as well the Emitter interface should explicitly mention that the methods should be called non-concurrently. Generate can be found in both Flowable and Observable.

generate:

<p>
Note that the {@link Emitter#onNext}, {@link Emitter#onError} and 
{@link Emitter#onComplete} methods provided to the function should be called synchronously,
never concurrently and only while the method is executing. Calling them from multiple threads 
or outside the function call is not supported and leads to an undefined behavior.

Emitter:

<p>
Note that the {@link Emitter#onNext}, {@link Emitter#onError} and 
{@link Emitter#onComplete} methods provided to the function should be called synchronously,
never concurrently. Calling them from multiple threads is not supported and leads to an 
undefined behavior.

[davidmoten]

I believe calling from multiple threads is supported if there is a happens-before relationship between calls. Perhaps this:

<p>
Note that there must be a <i>happens-before</i> relationship between calls to the {@link Emitter#onNext}, {@link Emitter#onError} and 
{@link Emitter#onComplete} methods. Calling those methods from multiple threads without establishing that relationship is not supported and leads to undefined behavior.

[akarnokd]

Fundamentally, the calls should not escape the callback method and one would have to block the callback if the onNext happens asynchronously to the method. Still you can only call it once and the blocking implicitly establishes this relationship.

[davidmoten]

Sounds good, thanks @akarnokd