kernel-doc: improve handling of whitespace on the first line param description

kernel-doc: improve handling of whitespace on the first line param description

Handle whitespace on the first line of param text as if it was the empty
string. There is no need to add the newline in this case. This improves
the rst output in particular, where blank lines may be problematic in
parameter lists.

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