kernel-doc: strip leading whitespace from continued param descs

If a param description spans multiple lines, check any leading
whitespace in the first continuation line, and remove same amount of
whitespace from following lines.

This allows indentation in the multi-line parameter descriptions for
aesthetical reasons while not causing accidentally significant
indentation in the rst output.

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