kernel-doc/rst: do not output DOC: section titles for requested ones

If the user requests a specific DOC: section by name, do not output its
section title. In these cases, the surrounding context already has a
heading, and the DOC: section title is only used as an identifier and a
heading for clarity in the source file.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>