doc: Add referential actions to CREATE/ALTER TABLE synopsis

The general constraint synopsis references "referential_action", but
this was not further defined in the synopsis section.  Compared to the
level of detail that the synopsis gives to other subclauses, this
should surely be there.

extracted from a patch by Paul Martinez <hellopfm@gmail.com>

Discussion: https://www.postgresql.org/message-id/flat/CACqFVBZQyMYJV=njbSMxf+rbDHpx=W=B7AEaMKn8dWn9OZJY7w@mail.gmail.com

doc/src/sgml/ref/alter_table.sgml

@@ -135,6 +135,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
 <phrase><replaceable class="parameter">exclude_element</replaceable> in an <literal>EXCLUDE</literal> constraint is:</phrase>
 
 { <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ <replaceable class="parameter">opclass</replaceable> ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]
+
+<phrase><replaceable class="parameter">referential_action</replaceable> in a <literal>FOREIGN KEY</literal>/<literal>REFERENCES</literal> constraint is:</phrase>
+
+{ NO ACTION | RESTRICT | CASCADE | SET NULL | SET DEFAULT }
 </synopsis>
  </refsynopsisdiv>
 

doc/src/sgml/ref/create_table.sgml

@@ -105,6 +105,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
 <phrase><replaceable class="parameter">exclude_element</replaceable> in an <literal>EXCLUDE</literal> constraint is:</phrase>
 
 { <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ <replaceable class="parameter">opclass</replaceable> ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]
+
+<phrase><replaceable class="parameter">referential_action</replaceable> in a <literal>FOREIGN KEY</literal>/<literal>REFERENCES</literal> constraint is:</phrase>
+
+{ NO ACTION | RESTRICT | CASCADE | SET NULL | SET DEFAULT }
 </synopsis>
 
  </refsynopsisdiv>