Doc: fix confusion about LEAKPROOF in syntax summaries.

The syntax summaries for CREATE FUNCTION and allied commands
made it look like LEAKPROOF is an alternative to
IMMUTABLE/STABLE/VOLATILE, when of course it is an orthogonal
option.  Improve that.

Per gripe from aazamrafeeque0.  Thanks to David Johnston for
suggestions.

Discussion: https://postgr.es/m/162444349581.694.5818572718530259025@wrigleys.postgresql.org

doc/src/sgml/ref/alter_function.sgml

@@ -35,7 +35,8 @@ ALTER FUNCTION <replaceable>name</replaceable> [ ( [ [ <replaceable class="param
 <phrase>where <replaceable class="parameter">action</replaceable> is one of:</phrase>
 
     CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
-    IMMUTABLE | STABLE | VOLATILE | [ NOT ] LEAKPROOF
+    IMMUTABLE | STABLE | VOLATILE
+    [ NOT ] LEAKPROOF
     [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
     PARALLEL { UNSAFE | RESTRICTED | SAFE }
     COST <replaceable class="parameter">execution_cost</replaceable>

doc/src/sgml/ref/alter_routine.sgml

@@ -34,7 +34,8 @@ ALTER ROUTINE <replaceable>name</replaceable> [ ( [ [ <replaceable class="parame
 
 <phrase>where <replaceable class="parameter">action</replaceable> is one of:</phrase>
 
-    IMMUTABLE | STABLE | VOLATILE | [ NOT ] LEAKPROOF
+    IMMUTABLE | STABLE | VOLATILE
+    [ NOT ] LEAKPROOF
     [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
     PARALLEL { UNSAFE | RESTRICTED | SAFE }
     COST <replaceable class="parameter">execution_cost</replaceable>

doc/src/sgml/ref/create_function.sgml

@@ -28,9 +28,10 @@ CREATE [ OR REPLACE ] FUNCTION
   { LANGUAGE <replaceable class="parameter">lang_name</replaceable>
     | TRANSFORM { FOR TYPE <replaceable class="parameter">type_name</replaceable> } [, ... ]
     | WINDOW
-    | IMMUTABLE | STABLE | VOLATILE | [ NOT ] LEAKPROOF
-    | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
-    | [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
+    | { IMMUTABLE | STABLE | VOLATILE }
+    | [ NOT ] LEAKPROOF
+    | { CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT }
+    | { [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER }
     | PARALLEL { UNSAFE | RESTRICTED | SAFE }
     | COST <replaceable class="parameter">execution_cost</replaceable>
     | ROWS <replaceable class="parameter">result_rows</replaceable>