util: Document wl_array and wl_list members

Despite their clear names, wl_array and wl_list members are undocumented,
resulting in doxygen warnings[1] when building documentation.

Document these members, suppressing the warnings.

[1] Warnings are visible when EXTRACT_ALL = NO.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>