Remove documentation that fatal cannot be rescued [ci skip]

You can rescue it:

f = ObjectSpace.each_object(Class){|c| break c if c.name == 'fatal'}
begin
  raise f
rescue f
  2
end # => 2

It's not a good idea to rescue fatal exceptions you didn't generate
yourself, though.

Fixes [Bug #10691]

error.c

@@ -2360,7 +2360,7 @@ syserr_eqq(VALUE self, VALUE exc)
  * Document-class: fatal
  *
  * fatal is an Exception that is raised when Ruby has encountered a fatal
- * error and must exit.  You are not able to rescue fatal.
+ * error and must exit.
  */
 
 /*