doc: add IPC callback limitations
doc: add IPC callback limitations

For asynchronous requests, user callback may be triggered either from
IPC thread or from interrupt thread. Because of this, delivery of
other interrupt-based events such as alarms may not be possible inside
the asynchronous IPC request callback handler. Document this
limitation.

Signed-off-by: Anatoly Burakov <814e00d8f6e4e81b7ccc7e3e5a0bfec0078f0203@intel.com>

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