docs/os/core_os/callout: Update docs

Removes deprecated part of API from the documentation entry.

docs/os/core_os/callout/callout.rst

@@ -9,23 +9,13 @@ Description
 Callout is a way of setting up an OS timer. When the timer fires, it is
 delivered as an event to task's event queue.
 
-User would initialize their callout structure using
-:c:func:`os_callout_init()`, or :c:func:`os_callout_func_init()` and 
-then arm it with :c:func:`os_callout_reset()`.
+User would initialize their callout structure :c:type:`struct os_callout` using
+:c:func:`os_callout_init()` and then arm it with :c:func:`os_callout_reset()`.
 
 If user wants to cancel the timer before it expires, they can either use
 :c:func:`os_callout_reset()` to arm it for later expiry, or stop it altogether
 by calling :c:func:`os_callout_stop()`.
 
-There are 2 different options for data structure to use. First is
-:c:type:`struct os_callout`, which is a bare-bones version. You would
-initialize this with :c:func:`os_callout_init()`.
-
-Second option is :c:type:`struct os_callout_func`. This you can use if you
-expect to have multiple different types of timers in your task, running
-concurrently. The structure contains a function pointer, and you would
-call that function from your task's event processing loop.
-
 Time unit when arming the timer is OS ticks. This rate of this ticker
 depends on the platform this is running on. You should use OS define
 ``OS_TICKS_PER_SEC`` to convert wallclock time to OS ticks.