docs: Add request_irq() documentation

While checking the results of the :c:func: removal, I noticed that there
was no documentation for request_irq(), and request_threaded_irq() was not
mentioned at all.  Add a kerneldoc comment for request_irq() and add
request_threaded_irq() to the list of functions.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>