os: callout without event queue (doc vs code)

Callout documentation makes it believe that an event queue is required to process callout callback, this requirement is not present in the code. If no queue is specified callback is processed in interupt context. Doc will need to be updated accordingly.
https://github.com/apache/mynewt-core/blob/e0d46d6f58bcc81a74536591e03560458f349b14/kernel/os/src/os_callout.c#L128-L132

Documentation also make mention of `OS_EVENT_T_TIMER` which is not defined anywhere

Start with kernel/os/src/os_callout.c at lines 128-132, then locate the callout documentation that describes callback processing. Verify the documentation against the code, clarify behavior when no event queue is specified, and remove or correct the reference to the undefined OS_EVENT_T_TIMER; done when the documentation matches the implementation.

Written by the indexing model from the issue text.