docs: reconcile thread_pool destructor reference wording with the manual #404

From the review result:

thread_pool::destructor --> "Signals all worker threads to stop, waits for them to finish, and destroys any pending work items". I couldn't figure out what this means. Obviously it's calling the base class destructor which does shutdown() destroy sequence -- but that didn't clarify either.

Source: Boost review result (conditional acceptance), Boost mailing list, September 2026.

The quoted sentence is still at include/boost/capy/ex/thread_pool.hpp:56. The clear explanation ("~thread_pool calls stop(), which abandons any work that has not yet started, then joins the worker threads") exists only in the hello-task example page, so the reference and the manual now say different things.

• Rewrite the destructor javadoc to state the stop/abandon/join sequence and its interaction with join()