doc: Minor corrections to kernel API documentation

Addresses a range of issues affecting the Kernel Primer or
the API Guide generated from doxygen tags.

* Ensures mailbox examples use kernel APIs correctly.
  (Fix for ZEP-1262, as well as other errors).

* Ensures memory alignment limitations for memory slabs
  are correctly described. (Fix for ZEP-1265.)

* Ensures memory alignment limitations for memory pools
  are more clearly described. Also fixes a typo in a
  memory pool example.

* Ensures memory alignment limitations for message
  queues are more clearly described.

* Fixes references to a number of kernel configuration
  options that were omitted or incorrectly formatted.

* Fixes a typo in an example of thread spawning.

Change-Id: I395186f333490b1e0c4223b87c0fe7136548770f
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>