kernel-doc: do not warn about duplicate default section names

kernel-doc: do not warn about duplicate default section names

Since

commit 3e2e430ce93759319b58303bdf9cf67765e6d045
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Sun May 29 09:40:44 2016 +0300

    kernel-doc: concatenate contents of colliding sections

we started getting (more) errors on duplicate section names, especially
on the default section name "Description":

include/net/mac80211.h:3174: warning: duplicate section name 'Description'

This is usually caused by a slightly unorthodox placement of parameter
descriptions, like in the above case, and kernel-doc resetting back to
the default section more than once within a kernel-doc comment.

Ignore warnings on the duplicate section name automatically assigned by
kernel-doc, and only consider explicitly user assigned duplicate section
names an issue.

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