doc: Fix up API descriptions for kernel event logger

Prepares the kernel event logger APIs for inclusion in the
API guide. Also corrects a couple of other issues:

* Gets rid of obsolete thread monitor code.
* Renames "timer_func" global variable to "_sys_k_timer_func"
  to align it with kernel naming conventions.

Change-Id: I93d403f83ae44ff45dda489c2ead7bfec6ce1fa3
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>