Doc: fix missing/bogus documentation of range_intersect_agg().

The form taking anymultirange had not been documented.  This was
fixed in HEAD in b21c4cf, but that should have been back-patched
to v14 since the function was added there.  Do so now.

Also, the form taking anyrange was incorrectly documented as
returning anymultirange, when it returns anyrange.

Remove b21c4cf from the v15 release notes, since it no longer
qualifies as new-in-v15.

Noted by Shay Rojansky.

Discussion: https://postgr.es/m/CADT4RqAktzP7t6SFf0Xqm9YhahzvsmxFbzXe-gFOd=+_CHm0JA@mail.gmail.com

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -21067,7 +21067,7 @@ SELECT NULLIF(value, '(none)') ...
         </indexterm>
         <function>range_intersect_agg</function> ( <parameter>value</parameter>
          <type>anyrange</type> )
-        <returnvalue>anymultirange</returnvalue>
+        <returnvalue>anyrange</returnvalue>
        </para>
        <para role="func_signature">
         <function>range_intersect_agg</function> ( <parameter>value</parameter>

doc/src/sgml/release-15.sgml

@@ -2489,17 +2489,6 @@ Add documentation for pg_encoding_to_char() and pg_char_to_encoding() (Ian Lawre
 </para>
 </listitem>
 
-<!--
-Author: Peter Eisentraut <peter@eisentraut.org>
-2022-03-30 [b21c4cf95] doc: Document range_intersect_agg(anymultirange)
--->
-
-<listitem>
-<para>
-Add documentation for range_intersect_agg(anymultirange) (Paul Jungwirth)
-</para>
-</listitem>
-
 <!--
 Author: Tom Lane <tgl@sss.pgh.pa.us>
 2021-10-09 [2ae5d72f0] Doc: improve documentation for ^@ starts-with operator.