doc: Fix section of functions age(xid) and mxid_age(xid)

In 17~, age(xid) and mxid_age(xid) were listed as deprecated.  Based on
the discussion that led to 48b5aa3, this is not intentional as this
could break many existing monitoring queries.  Note that vacuumdb also
uses both of them.

In 16, both functions were listed under "Control Data Functions", which
is incorrect, so let's move them to the list of functions related to
transaction IDs and snapshots.

Author: Bertrand Drouvot
Discussion: https://postgr.es/m/Zzr2zZFyeFKXWe8a@ip-10-97-1-34.eu-west-3.compute.internal
Discussion: https://postgr.es/m/20231114013224.4z6oxa6p6va33rxr@awork3.anarazel.de
Backpatch-through: 16

Author: michaelpq

doc/src/sgml/func.sgml

@@ -27138,6 +27138,34 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
      </thead>
 
      <tbody>
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>age</primary>
+        </indexterm>
+        <function>age</function>  ( <type>xid</type> )
+        <returnvalue>integer</returnvalue>
+       </para>
+       <para>
+        Returns the number of transactions between the supplied
+        transaction id and the current transaction counter.
+       </para></entry>
+      </row>
+
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>mxid_age</primary>
+        </indexterm>
+        <function>mxid_age</function>  ( <type>xid</type> )
+        <returnvalue>integer</returnvalue>
+       </para>
+       <para>
+        Returns the number of multixacts IDs between the supplied
+        multixact ID and the current multixacts counter.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
@@ -27282,7 +27310,8 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
    <para>
     The internal transaction ID type <type>xid</type> is 32 bits wide and
     wraps around every 4 billion transactions.  However,
-    the functions shown in <xref linkend="functions-pg-snapshot"/> use a
+    the functions shown in <xref linkend="functions-pg-snapshot"/>, except
+    <function>age</function> and <function>mxid_age</function>, use a
     64-bit type <type>xid8</type> that does not wrap around during the life
     of an installation and can be converted to <type>xid</type> by casting if
     required;  see <xref linkend="transaction-id"/> for details.
@@ -27364,33 +27393,6 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
      </thead>
 
      <tbody>
-      <row>
-       <entry role="func_table_entry"><para role="func_signature">
-        <indexterm>
-         <primary>age</primary>
-        </indexterm>
-        <function>age</function>  ( <type>xid</type> )
-        <returnvalue>integer</returnvalue>
-       </para>
-       <para>
-        Returns the number of transactions between the supplied
-        transaction id and the current transaction counter.
-       </para></entry>
-      </row>
-
-      <row>
-       <entry role="func_table_entry"><para role="func_signature">
-        <indexterm>
-         <primary>mxid_age</primary>
-        </indexterm>
-        <function>mxid_age</function>  ( <type>xid</type> )
-        <returnvalue>integer</returnvalue>
-       </para>
-       <para>
-        Returns the number of multixacts IDs between the supplied
-        multixact ID and the current multixacts counter.
-       </para></entry>
-      </row>
 
       <row>
        <entry role="func_table_entry"><para role="func_signature">