doc: add API docs in elm_interface_scrollable.

doc: add API docs in elm_interface_scrollable.

This adds
- gravity_set
- gravity_get
- bounce_allow_set
- bounce_allow_get
- movement_block_set
- movement_block_get
- policy_get
- policy_set
- content_region_get
- content_region_set
- page_size_set
- page_size_get
- page_snap_allow_set
- page_snap_allow_get
- single_direction_get
- single_direction_set
- last_page_get
- current_page_get
- content_size_get

- scroll_up_cb
- hbar_drag_cb
- drag_start_cb
- scroll_left_cb
- vbar_press_cb
- hbar_press_cb
- hbar_unpress_cb
- drag_stop_cb
- page_change_cb
- animate_start_cb
- scroll_down_cb
- scroll_cb
- animate_stop_cb
- scroll_right_cb
- edge_left_cb
- vbar_drag_cb
- vbar_unpress_cb
- edge_bottom_cb
- edge_top_cb

- page_show
- region_bring_in
- page_bring_in
- content_region_show
- content_min_limit