Add clarification to docs on ASYNC_free_pool()

ASYNC_init_pool(). If the user application is multi-threaded, then this should

be done for each thread that will initiate asynchronous jobs. Before user code

exits it should free the pool up (for each thread where a pool was initialised)

using ASYNC_free_pool(). The B<max_size> argument limits the number of

ASYNC_JOBs that will be held in the pool. If B<max_size> is set to 0 then no

upper limit is set. When an ASYNC_JOB is needed but there are none available in

the pool already then one will be automatically created, as long as the total

of ASYNC_JOBs managed by the pool does not exceed B<max_size>. When the pool is

first initialised B<init_size> ASYNC_JOBs will be created immediately. If

ASYNC_init_pool() is not called before the pool is first used then it will be

called automatically with a B<max_size> of 0 (no upper limit) and an

B<init_size> of 0 (no ASYNC_JOBs created up front). If a pool is created in this

way it must still be cleaned up with an explicit call to ASYNC_free_pool().

using ASYNC_free_pool(). No asynchronous jobs must be outstanding for the thread

when ASYNC_free_pool() is called. Failing to ensure this will result in memory

leaks.

The B<max_size> argument limits the number of ASYNC_JOBs that will be held in

the pool. If B<max_size> is set to 0 then no upper limit is set. When an

ASYNC_JOB is needed but there are none available in the pool already then one

will be automatically created, as long as the total of ASYNC_JOBs managed by the

pool does not exceed B<max_size>. When the pool is first initialised

B<init_size> ASYNC_JOBs will be created immediately. If ASYNC_init_pool() is not

called before the pool is first used then it will be called automatically with a

B<max_size> of 0 (no upper limit) and an B<init_size> of 0 (no ASYNC_JOBs

created up front). If a pool is created in this way it must still be cleaned up

with an explicit call to ASYNC_free_pool().

An asynchronous job is started by calling the ASYNC_start_job() function.

Initially B<*job> should be NULL. B<ret> should point to a location where the