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()

src/hb-ot-layout.cc

diff --git a/src/hb-ot-layout.cc b/src/hb-ot-layout.cc
index 8c91842..aa36954 100644
--- a/src/hb-ot-layout.cc
+++ b/src/hb-ot-layout.cc
@@ -373,6 +373,11 @@
   return hb_ot_layout_table_select_script (face, table_tag, t - script_tags, script_tags, script_index, chosen_script);
 }
 
+/**
+ * hb_ot_layout_table_select_script:
+ *
+ * Since: REPLACEME
+ **/
 hb_bool_t
 hb_ot_layout_table_select_script (hb_face_t      *face,
 				  hb_tag_t        table_tag,
@@ -481,6 +486,11 @@
   return hb_ot_layout_script_select_language (face, table_tag, script_index, 1, &language_tag, language_index);
 }
 
+/**
+ * hb_ot_layout_script_select_language:
+ *
+ * Since: REPLACEME
+ **/
 hb_bool_t
 hb_ot_layout_script_select_language (hb_face_t      *face,
 				     hb_tag_t        table_tag,