doc: fix warnings due to :c:option: and :option:

:c:option:`xyz` is non usable without a . c:option:: declaration, so moved
to *xyz`; likewise :option:`xyz` where no .. option:: xyz is declared.

Change-Id: I011ccf2aac244125dbe2d09d197e443bd4c12fe2
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>