doc: Revise timer example to use workqueue instead of alert

Rewrites the example of a timer's expiry routine offloading
processing that can't be done at interrupt level. The example
now submits work to the system workqueue directly, rather than
using an alert. This saves footprint by eliminating the need
for alert-related API support that isn't needed. (This is a
true savings, since the alert code just called the same
workqueue APIs the example now calls directly.)

Change-Id: I378e40aef33014f2c75c4f57531f75247d50e479
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>