[io grant] Expand IO::Path.relative

- include info on what happens if the path is already relative

doc/Type/IO/Path.pod6

@@ -151,9 +151,12 @@ created with if it is not.
 
 =head2 method relative
 
-    method relative(IO::Path:D: $base = ~$*CWD --> Str)
+    method relative(IO::Path:D: $base = $*CWD --> Str)
 
-Returns a new C<Str> object relative to the C<$base> path.
+Returns a new C<Str> object with the path relative to the C<$base>. If C<$base>
+is not provided, C<$*CWD> is used in its place. If the invocant is not
+an absolute path, it's first made to be absolute using the C<.CWD>
+attribute the object was created with, and then is made relative to C<$base>.
 
 =head2 method parent