In signatures.rakudoc, remark that "invocant marker" gets explained in the next section

This is the first time the term "invocant marker" is used in this document, so let readers know that they'll find it explained immediately hereafter.

Author: schultzdavid

doc/Language/signatures.rakudoc

@@ -99,7 +99,7 @@ L<C<Signature>|/type/Signature> literals can contain string/numeric literals
     my $sig = :('Þor', Str, Int);
     say <Þor Hammer 1> ~~ $sig; # OUTPUT: «True␤»
 
-And they can also contain the invocant marker
+And they can also contain the invocant marker, explained in the next section:
 
     class Foo {
         method bar( $self: ){ "baz" }