Add doc stubs for recently added API

Add doc stubs for recently added API

Thanks to David Corbett who revamped our script and language processing
and implemented full BCP 47 support.

https://github.com/harfbuzz/harfbuzz/pull/730

New API:
+hb_ot_layout_table_select_script()
+hb_ot_layout_script_select_language()
+HB_OT_MAX_TAGS_PER_SCRIPT
+HB_OT_MAX_TAGS_PER_LANGUAGE
+hb_ot_tags_from_script_and_language()
+hb_ot_tags_to_script_and_language()

Deprecated API:
-hb_ot_layout_table_choose_script()
-hb_ot_layout_script_find_language()
-hb_ot_tags_from_script()
-hb_ot_tag_from_language()