doc: Edit microkernel_mutexes.rst; add ReST syntax, clarifications.

Making all the files in this section consistent with :dfn: in
the intro paragraph, API headings, present-tense verbs in APIs.
Clarified paragraphs on priorty-bazed waiting and priority
inheritance.

Change-Id: I26cc49926bc49c9c68300b6249aeb52c1be33625
Signed-off-by: L.S. Cook <leonax.cook@intel.com>