Add documentation of systimestamp and sysdate in sql manual

We support both systimestamp and sysdate but they are no documented there.

Systimestamp is equal to current_timestamp, and sysdate is equal to current_date.

 

>>select systimestamp from dual;

 

(EXPR)                   

--------------------------

 

2018-03-10 10:32:19.232949

 

— 1 row(s) selected.

>>select sysdate from dual;

 

(EXPR)   

----------

 

2018-03-10

 

— 1 row(s) selected.

>>select current_timestamp from dual;

 

(EXPR)                    

--------------------------

 

2018-03-10 10:32:30.733581

 

— 1 row(s) selected.

>>select current_date from dual;

 

(EXPR)   

----------

 

2018-03-10

 

— 1 row(s) selected.