doc: Edit microkernel_fifos for consistent .rst style and formatting.

Making all the files in this section consistent with :dfn: in the intro
paragraph, API headings, present-tense verbs in APIs, parallel wording.

Change-Id: I5259c443076b1ac6602352dab42d35d5aca6e5b5
Signed-off-by: L.S. Cook <leonax.cook@intel.com>