docs: update concurrency.md regarding thread creation

First, spdk_thread is an abstraction for a thread of execution and

spdk_poller is an abstraction for a function that should be

periodically called on the given thread. On each system thread that the user

wishes to use with SPDK, they must first call spdk_allocate_thread(). This

function takes three function pointers - one that will be called to pass a

message to this thread, one that will be called to request that a poller be

started on this thread, and finally one to request that a poller be stopped.

*The implementation of these functions is not provided by this library*. Many

applications already have facilities for passing messages, so to ease

integration with existing code bases we've left the implementation up to the

user. However, for users starting from scratch, see the following section on

the event framework for an SPDK-provided implementation.

wishes to use with SPDK, they must first call spdk_thread_create().

The library also defines two other abstractions: spdk_io_device and

spdk_io_channel. In the course of implementing SPDK we noticed the

As the number of example applications in SPDK grew, it became clear that a

large portion of the code in each was implementing the basic message passing

infrastructure required to call spdk_allocate_thread(). This includes spawning

infrastructure required to call spdk_thread_create(). This includes spawning

one thread per core, pinning each thread to a unique core, and allocating

lockless rings between the threads for message passing. Instead of

re-implementing that infrastructure for each example application, SPDK

message passing infrastructure, installing signal handlers to cleanly

shutdown, implements periodic pollers, and does basic command line parsing.

When started through spdk_app_start(), the library automatically spawns all of

the threads requested, pins them, and calls spdk_allocate_thread() with

appropriate function pointers for each one. This makes it much easier to

implement a brand new SPDK application and is the recommended method for those

starting out. Only established applications with sufficient message passing

infrastructure should consider directly integrating the lower level libraries.

the threads requested, pins them, and calls spdk_thread_create(). This makes

it much easier to implement a brand new SPDK application and is the recommended

method for those starting out. Only established applications with sufficient

message passing infrastructure should consider directly integrating the lower

level libraries.

# Limitations of the C Language