doc: Add description of some missing monitoring functions

This commit adds some documentation about two monitoring functions:
- pg_stat_get_xact_blocks_fetched()
- pg_stat_get_xact_blocks_hit()

The description of these functions has been removed in ddfc2d9, later
simplified by 5f2b089, assuming that all the functions whose
descriptions were removed are used in system views.  Unfortunately, some
of them were are not used in any system views, so they lacked
documentation.

This gap exists in the docs for a long time, so backpatch all the way
down.

Reported-by: Michael Paquier
Author: Bertrand Drouvot
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/ZBeeH5UoNkTPrwHO@paquier.xyz
Backpatch-through: 11

doc/src/sgml/monitoring.sgml

@@ -3142,6 +3142,24 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
        Discard the current statistics snapshot
       </entry>
      </row>
+     
+     <row>
+      <entry><literal><function>pg_stat_get_xact_blocks_fetched</function>(oid)</literal><indexterm><primary>pg_stat_get_xact_blocks_fetched</primary></indexterm></entry>
+      <entry><type>bigint</type></entry>
+      <entry>
+       Returns the number of buffers fetched for table or index, in the current
+       transaction.
+      </entry>
+     </row>
+
+     <row>
+      <entry><literal><function>pg_stat_get_xact_blocks_hit</function>(oid)</literal><indexterm><primary>pg_stat_get_xact_blocks_hit</primary></indexterm></entry>
+      <entry><type>bigint</type></entry>
+      <entry>
+       Returns the number of buffer hits for table or index, in the current
+       transaction.
+      </entry>
+     </row>
 
      <row>
       <entry><literal><function>pg_stat_reset()</function></literal><indexterm><primary>pg_stat_reset</primary></indexterm></entry>