Add to DateTime.later/.earlier

- Note behaviour with leap seconds
    spec: Raku/roast@66d29b7aa8
- Note negatives work with `earlier`
- Change links to work on routine-specific pages too, not just the
    type pages

Author: zoffixznet

doc/Type/DateTime.pod6

@@ -281,8 +281,13 @@ unit may be passed.
     say $d.later(days => 2).later(:1month);  # OUTPUT: «2015-04-01T00:00:00Z␤»
     say $d.later(days => 2).later(:month);   # same, as +True === 1
 
-Negative offsets are allowed, though L<#method earlier> is more idiomatic for
-that.
+If the resultant time has value C<60> for seconds, yet no leap second
+actually exists for that time, seconds will be set to C<59>:
+
+    say DateTime.new('2008-12-31T23:59:60Z').later: :1day;
+    # OUTPUT: «2009-01-01T23:59:59Z␤»
+
+Negative offsets are allowed, though L<earlier> is more idiomatic for that.
 
 =head2 method earlier
 
@@ -296,6 +301,14 @@ towards the past applied. See L<#method later> for usage.
     my $d = DateTime.new(date => Date.new('2015-02-27'));
     say $d.earlier(month => 1).earlier(:2days);  # OUTPUT: «2015-01-25T00:00:00Z␤»
 
+If the resultant time has value C<60> for seconds, yet no leap second
+actually exists for that time, seconds will be set to C<59>:
+
+    say DateTime.new('2008-12-31T23:59:60Z').earlier: :1day;
+    # OUTPUT: «2008-12-30T23:59:59Z␤»
+
+Negative offsets are allowed, though L<later> is more idiomatic for that.
+
 =head2 method truncated-to
 
 Defined as: