Doc: improve introductory information about procedures.

Clarify the discussion in "User-Defined Procedures", by laying out
the key differences between functions and procedures in a bulleted
list.  Notably, this avoids burying the lede about procedures being
able to do transaction control.  Make the back-link in the CREATE
FUNCTION reference page more prominent, and add one in CREATE
PROCEDURE.

Per gripe from Guyren Howe.  Thanks to David Johnston for discussion.

Discussion: https://postgr.es/m/BYAPR03MB4903C53A8BB7EFF5EA289674A6949@BYAPR03MB4903.namprd03.prod.outlook.com

doc/src/sgml/ref/create_function.sgml

@@ -100,6 +100,11 @@ CREATE [ OR REPLACE ] FUNCTION
    To be able to create a function, you must have <literal>USAGE</literal>
    privilege on the argument types and the return type.
   </para>
+
+  <para>
+   Refer to <xref linkend="xfunc"/> for further information on writing
+   functions.
+  </para>
  </refsect1>
 
  <refsect1>
@@ -578,12 +583,6 @@ CREATE [ OR REPLACE ] FUNCTION
     </varlistentry>
 
    </variablelist>
-
-   <para>
-    Refer to <xref linkend="xfunc"/> for further information on writing
-    functions.
-   </para>
-
  </refsect1>
 
  <refsect1 id="sql-createfunction-overloading">
@@ -661,8 +660,7 @@ CREATE FUNCTION foo(int, int default 42) ...
   <title>Examples</title>
 
   <para>
-   Here are some trivial examples to help you get started.  For more
+   Add two integers using a SQL function:
-   information and examples, see <xref linkend="xfunc"/>.
 <programlisting>
 CREATE FUNCTION add(integer, integer) RETURNS integer
     AS 'select $1 + $2;'

doc/src/sgml/ref/create_procedure.sgml

@@ -76,6 +76,11 @@ CREATE [ OR REPLACE ] PROCEDURE
    To be able to create a procedure, you must have <literal>USAGE</literal>
    privilege on the argument types.
   </para>
+
+  <para>
+   Refer to <xref linkend="xproc"/> for further information on writing
+   procedures.
+  </para>
  </refsect1>
 
  <refsect1>

doc/src/sgml/xfunc.sgml

@@ -81,21 +81,53 @@
   </indexterm>
 
    <para>
-    A procedure is a database object similar to a function.  The difference is
+    A procedure is a database object similar to a function.
-    that a procedure does not return a value, so there is no return type
+    The key differences are:
-    declaration.  While a function is called as part of a query or DML
+
-    command, a procedure is called in isolation using
+    <itemizedlist>
-    the <xref linkend="sql-call"/> command.  If the CALL command is not
+     <listitem>
-    part of an explicit transaction, a procedure in many server-side
+      <para>
-    languages can commit, rollback, and begin new transactions during
+       Procedures are defined with the <xref linkend="sql-createprocedure"/>
-    its execution, which is not possible in functions.
+       command, not <command>CREATE FUNCTION</command>.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       Procedures do not return a function value; hence <command>CREATE
+       PROCEDURE</command> lacks a <literal>RETURNS</literal> clause.
+       However, procedures can instead return data to their callers via
+       output parameters.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       While a function is called as part of a query or DML command, a
+       procedure is called in isolation using
+       the <xref linkend="sql-call"/> command.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       A procedure can commit or roll back transactions during its
+       execution (then automatically beginning a new transaction), so long
+       as the invoking <command>CALL</command> command is not part of an
+       explicit transaction block.  A function cannot do that.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       Certain function attributes, such as strictness, don't apply to
+       procedures.  Those attributes control how the function is
+       used in a query, which isn't relevant to procedures.
+      </para>
+     </listitem>
+    </itemizedlist>
    </para>
 
    <para>
-    The explanations on how to define user-defined functions in the rest of
+    The explanations in the following sections about how to define
-    this chapter apply to procedures as well, except that
+    user-defined functions apply to procedures as well, except for the
-    the <xref linkend="sql-createprocedure"/> command is used instead, there is
+    points made above.
-    no return type, and some other features such as strictness don't apply.
    </para>
 
    <para>