doc: remove reference to task IRQs, deprecated

Per:

commit a83f895d
Author: Andrew Boie <andrew.p.boie@intel.com>
Commit: Benjamin Walsh <benjamin.walsh@windriver.com>

    microkernel: deprecate task IRQs

    This mechanism does not add enough value to the kernel to be worth
    maintaining it. Drivers that need deferred processing of interrupts
    can simply define their own task and have the interrupt handler
    release an event that the task waits on.

    The API is marked as deprecated and it is removed from unit test
    coverage as well as the documentation.

    Change-Id: Ib87b91cb41e9b6d7fdf0dc62b240a531b6a8889f
    Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

Change-Id: I0d891772b1fb4246ec9ee9f4764b2121333ae972
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>