Document DateTime.later/.earlier .Intify units…

…unless the unit is `second`/`seconds`. Intification was being
done for awhile in rakudo and was removed from `second` units
in rakudo/rakudo@656ff77b96
Propspec: Raku/roast@7c3bf500ed

doc/Type/DateTime.pod6

@@ -268,6 +268,9 @@ Returns a DateTime object based on the current one, but with a time delta
 applied. The time delta can be passed as a named argument where the argument
 name is the unit.
 
+Unless the given unit is C<second> or C<seconds>, the given value will be
+converted to an L<Int>.
+
 Allowed units are C<second>, C<seconds>, C<minute>, C<minutes>, C<hour>,
 C<hours>, C<day>, C<days>, C<week>, C<weeks>, C<month>, C<months>, C<year>,
 C<years>. Please note that the plural forms can only be used with
@@ -301,7 +304,8 @@ Defined as:
     method earlier(DateTime:D: *%unit)
 
 Returns a DateTime object based on the current one, but with a time delta
-towards the past applied. See L<#method later> for usage.
+towards the past applied. Unless the given unit is C<second> or C<seconds>,
+the given value will be converted to an L<Int>. 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␤»