doc: Revise kernel event logger documentation

Makes the purpose and capabilities of the kernel event logger
clearer, and leaves much of the low-level detail relating to
use of the configuration options and APIs to the configuration
option guide and API guide, respectively. Also corrects some
bugs in the example code for retrieving event information.

Also updates the API guide to make a clear distinction between
the general purpose event logger framework and the kernel event
logger (which is a specific instance of this framework).

Change-Id: I924f65092b2b0e5050af13376b5da85a6cdc1a65
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>