doc: Update uses of the word "procedure"

Historically, the term procedure was used as a synonym for function in
Postgres/PostgreSQL.  Now we have procedures as separate objects from
functions, so we need to clean up the documentation to not mix those
terms.

In particular, mentions of "trigger procedures" are changed to "trigger
functions", and access method "support procedures" are changed to
"support functions".  (The latter already used FUNCTION in the SQL
syntax anyway.)  Also, the terminology in the SPI chapter has been
cleaned up.

A few tests, examples, and code comments are also adjusted to be
consistent with documentation changes, but not everything.

Reported-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Jonathan S. Katz <jonathan.katz@excoventures.com>

doc/src/sgml/ref/create_operator.sgml

@@ -94,7 +94,7 @@ CREATE OPERATOR <replaceable>name</replaceable> (
 
   <para>
    The <replaceable class="parameter">function_name</replaceable>
-   procedure must have been previously defined using <command>CREATE
+   function must have been previously defined using <command>CREATE
    FUNCTION</command> and must be defined to accept the correct number
    of arguments (either one or two) of the indicated types.
   </para>
@@ -264,11 +264,11 @@ COMMUTATOR = OPERATOR(myschema.===) ,
 CREATE OPERATOR === (
     LEFTARG = box,
     RIGHTARG = box,
-    PROCEDURE = area_equal_procedure,
+    PROCEDURE = area_equal_function,
     COMMUTATOR = ===,
     NEGATOR = !==,
-    RESTRICT = area_restriction_procedure,
-    JOIN = area_join_procedure,
+    RESTRICT = area_restriction_function,
+    JOIN = area_join_function,
     HASHES, MERGES
 );
 </programlisting></para>