Fix function signatures of pageinspect in documentation

tuple_data_split() lacked the type of the first argument, and
heap_page_item_attrs() has reversed the first and second argument,
with the bytea argument using an incorrect name.

Author: Laurenz Albe
Discussion: https://postgr.es/m/8f9ab7b16daf623e87eeef5203a4ffc0dece8dfd.camel@cybertec.at
Backpatch-through: 9.6

Author: michaelpq

doc/src/sgml/pageinspect.sgml

@@ -156,7 +156,7 @@ test=# SELECT * FROM heap_page_items(get_raw_page('pg_class', 0));
 
    <varlistentry>
     <term>
-     <function>tuple_data_split(rel_oid, t_data bytea, t_infomask integer, t_infomask2 integer, t_bits text [, do_detoast bool]) returns bytea[]</function>
+     <function>tuple_data_split(rel_oid oid, t_data bytea, t_infomask integer, t_infomask2 integer, t_bits text [, do_detoast bool]) returns bytea[]</function>
      <indexterm>
       <primary>tuple_data_split</primary>
      </indexterm>
@@ -181,7 +181,7 @@ test=# SELECT tuple_data_split('pg_class'::regclass, t_data, t_infomask, t_infom
 
    <varlistentry>
     <term>
-     <function>heap_page_item_attrs(rel_oid, t_data bytea, [, do_detoast bool]) returns bytea[]</function>
+     <function>heap_page_item_attrs(page bytea, rel_oid regclass, [, do_detoast bool]) returns setof record</function>
      <indexterm>
       <primary>heap_page_item_attrs</primary>
      </indexterm>