docs: Clarify a doc comment

Reword the docs for hb_ot_layout_script_select_language and
be a bit more specific about the returned index.

src/hb-ot-layout.cc

@@ -717,10 +717,14 @@ hb_ot_layout_script_find_language (hb_face_t    *face,
  * @language_tags: The array of language tags
  * @language_index: (out): The index of the requested language
  *
- * Fetches the index of a given language tag in the specified face's GSUB table
+ * Fetches the index of the first language tag fom @language_tags that is present
- * or GPOS table, underneath the specified script index.
+ * in the specified face's GSUB or GPOS table, underneath the specified script
+ * index.
  *
- * Return value: %true if the language tag is found, %false otherwise
+ * If none of the given language tags is found, %false is returned and
+ * @language_index is set to the default language index.
+ *
+ * Return value: %true if one of the given language tags is found, %false otherwise
  *
  * Since: 2.0.0
  **/