Doc: Update struct Trigger definition.

Commit 487e986 added a new field to struct Trigger, but failed to
update the documentation to match; backpatch to v13 where that came in.

Reviewed by Richard Guo.

Discussion: https://postgr.es/m/CAPmGK17NY92CyxJ%2BBG7A3JZurmng4jfRfzPiBTtNupGMF0xW1g%40mail.gmail.com

Author: Etsuro Fujita

doc/src/sgml/trigger.sgml

@@ -721,6 +721,7 @@ typedef struct Trigger
     int16       tgtype;
     char        tgenabled;
     bool        tgisinternal;
+    bool        tgisclone;
     Oid         tgconstrrelid;
     Oid         tgconstrindid;
     Oid         tgconstraint;