libweston: change windowed_output_api output_create to create_head

Rename the function pointer to create_head() because that is what it
does on backends that are converted to the head-based API. Update the
documentation to match.

Surprisingly this is not an ABI break, as the function behaviour and
signature remain intact. Hence API_NAME is not bumped.

This is only an API break, and main.c is fixed accordingly.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>