docs: kernel-doc: Rename and split STATE_FIELD

STATE_FIELD describes a parser state that can handle any part of a
kerneldoc comment body; rename it to STATE_BODY to reflect that.

The $in_purpose variable was a hidden substate of STATE_FIELD; get rid of
it and make a proper state (STATE_BODY_MAYBE) instead.  This will make the
subsequent process_file() splitup easier.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>