Javadoc: Noting changes in Executor availability check approaches (1.536)

This change is a follow-up to <code>Executor::isAlive()</code> issues, which I've experienced in unit tests for #1229.

Since 1.536, <code>Executors</code> start threads on-demand.
The entire logic should use <code>Executor::isActive()</code> instead of Executor::isAlive()</code> in order to check if the <code>Executor</code> it ready to take tasks.

Since the change breaks previous executor availability flows, it is required to document changes somehow.

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>