Add some terms from terms.pod

lib/Language/terms.pod

@@ -183,30 +183,30 @@ or as constants
     constant forty-two = 42;
     say fourty-two
 
-=head2 self
+=head2 term self
 
 Inside a method, C<self> refers to the invocant (i.e. the object the method
 was called on). If used in a context where it doesn't make sense, a
 compile-time exception of type L<X::Syntax::NoSelf> is thrown.
 
-=head2 now
+=head2 term now
 
 Returns an L<Instant> object representing the current time.
 
-=head2 rand
+=head2 term rand
 
 Returns a pseudo-random L<Num> in the range C<0..^1>.
 
-=head2 pi
+=head2 term pi
 
 Returns the number C<pi>, i.e. the ratio between circumference and diameter of
 a circle.
 
-=head2 e
+=head2 term e
 
 Returns Euler's number
 
-=head2 i
+=head2 term i
 
 Returns the imaginary unit (for L<Complex> numbers).