Reword Leap Second Blurb

Closes #768

Author: coke

doc/Type/Instant.pod6

@@ -20,9 +20,11 @@ other operations with Instants are undefined.
 
 =head1 Future Leap Seconds
 
-Perl 6's methods that involve knowledge of leap seconds do not
-make any guesses for leap seconds in the future and only use the already-known
-leap seconds. This means you can get different results, depending on the
+The methods that involve knowledge of leap seconds always assume
+that there will be no further leaps after the last leap second
+that the implementation knows about, which may not be the last
+leap second that has actually been scheduled.
+This means you can get different results, depending on the
 compiler version you're using. For example, the December 31, 2016 leap second
 was announced in July and shipped with Rakudo 2016.07, so 2016.06 and earlier
 releases won't know about it: