cursor: make param names match with documentation

cursor: make param names match with documentation

There was a mismatch here.

Use a good-looking function param name because that's what will
show up in docs. Use an abbreviation inside the function.

Fixes the following warnings:

    cursor/wayland-cursor.c:504: warning: argument 'cursor' of command @param is not found in the argument list of wl_cursor_frame(struct wl_cursor *_cursor, uint32_t time)
    cursor/wayland-cursor.c:504: warning: The following parameter of wl_cursor_frame(struct wl_cursor *_cursor, uint32_t time) is not documented:
      parameter '_cursor'
    cursor/wayland-cursor.c:452: warning: argument 'cursor' of command @param is not found in the argument list of wl_cursor_frame_and_duration(struct wl_cursor *_cursor, uint32_t time, uint32_t *duration)
    cursor/wayland-cursor.c:452: warning: The following parameter of wl_cursor_frame_and_duration(struct wl_cursor *_cursor, uint32_t time, uint32_t *duration) is not documented:
      parameter '_cursor'
    cursor/wayland-cursor.c:147: warning: argument 'image' of command @param is not found in the argument list of wl_cursor_image_get_buffer(struct wl_cursor_image *_img)
    cursor/wayland-cursor.c:147: warning: The following parameter of wl_cursor_image_get_buffer(struct wl_cursor_image *_img) is not documented:
      parameter '_img'
    cursor/wayland-cursor.c:504: warning: argument 'cursor' of command @param is not found in the argument list of wl_cursor_frame(struct wl_cursor *_cursor, uint32_t time)
    cursor/wayland-cursor.c:504: warning: The following parameter of wl_cursor_frame(struct wl_cursor *_cursor, uint32_t time) is not documented:
      parameter '_cursor'
    cursor/wayland-cursor.c:452: warning: argument 'cursor' of command @param is not found in the argument list of wl_cursor_frame_and_duration(struct wl_cursor *_cursor, uint32_t time, uint32_t *duration)
    cursor/wayland-cursor.c:452: warning: The following parameter of wl_cursor_frame_and_duration(struct wl_cursor *_cursor, uint32_t time, uint32_t *duration) is not documented:
      parameter '_cursor'
    cursor/wayland-cursor.c:147: warning: argument 'image' of command @param is not found in the argument list of wl_cursor_image_get_buffer(struct wl_cursor_image *_img)
    cursor/wayland-cursor.c:147: warning: The following parameter of wl_cursor_image_get_buffer(struct wl_cursor_image *_img) is not documented:
      parameter '_img'

Signed-off-by: Simon Ser <contact@emersion.fr>