Add to_regtypemod function to extract typemod from a string type name.

In combination with to_regtype, this allows converting a string to
the "canonicalized" form emitted by format_type.  That usage requires
parsing the string twice, which is slightly annoying but not really
too expensive.  We considered alternatives such as returning a record
type, but that way was notationally uglier than this, and possibly
less flexible.

Like to_regtype(), we'd rather that this return NULL for any bad
input, but the underlying type-parsing logic isn't yet capable of
not throwing syntax errors.  Adjust the documentation for both
functions to point that out.

In passing, fix up a couple of nearby entries in the System Catalog
Information Functions table that had not gotten the word about our
since-v13 convention for displaying function usage examples.

David Wheeler and Erik Wienhold, reviewed by Pavel Stehule, Jim Jones,
and others.

Discussion: https://postgr.es/m/DF2324CA-2673-4ABE-B382-26B5770B6AA3@justatheory.com

src/backend/utils/adt/regproc.c

@@ -1220,6 +1220,26 @@ to_regtype(PG_FUNCTION_ARGS)
 	PG_RETURN_DATUM(result);
 }
 
+/*
+ * to_regtypemod	- converts "typename" to type modifier
+ *
+ * If the name is not found, we return NULL.
+ */
+Datum
+to_regtypemod(PG_FUNCTION_ARGS)
+{
+	char	   *typ_name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	Oid			typid;
+	int32		typmod;
+	ErrorSaveContext escontext = {T_ErrorSaveContext};
+
+	/* We rely on parseTypeString to parse the input. */
+	if (!parseTypeString(typ_name, &typid, &typmod, (Node *) &escontext))
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT32(typmod);
+}
+
 /*
  * regtypeout		- converts type OID to "typ_name"
  */