kernel-doc: concatenate contents of colliding sections

If there are multiple sections with the same section name, the current
implementation results in several sections by the same heading, with the
content duplicated from the last section to all. Even if there's the
error message, a more graceful approach is to combine all the
identically named sections into one, with concatenated contents.

With the supported sections already limited to select few, there are
massively fewer collisions than there used to be, but this is still
useful for e.g. when function parameters are documented in the middle of
a documentation comment, with description spread out above and
below. (This is not a recommended documentation style, but used in the
kernel nonetheless.)

We can now also demote the error to a warning.

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