document forceRadial.x() and .y() as setting accessor functions inste…

…ad of numbers

README.md

@@ -453,8 +453,25 @@ The *radius* accessor is invoked for each [node](#simulation_nodes) in the simul
 
 <a name="radial_x" href="#radial_x">#</a> <i>radial</i>.<b>x</b>([<i>x</i>]) [<>](https://github.com/d3/d3-force/blob/master/src/radial.js "Source")
 
-If *x* is specified, sets the *x*-coordinate of the circle center to the specified number and returns this force. If *x* is not specified, returns the current *x*-coordinate of the center, which defaults to zero.
+If *x* is specified, sets the *x*-coordinate accessor to the specified number or function, re-evaluates the *x*-accessor for each node, and returns this force. If *x* is not specified, returns the current *x*-accessor, which defaults to:
+
+```js
+function x() {
+  return 0;
+}
+```
+
+The *x*-accessor is invoked for each [node](#simulation_nodes) in the simulation, being passed the *node* and its zero-based *index*. The resulting number is then stored internally, such that the target *x*-coordinate of each node is only recomputed when the force is initialized or when this method is called with a new *x*, and not on every application of the force.
 
 <a name="radial_y" href="#radial_y">#</a> <i>radial</i>.<b>y</b>([<i>y</i>]) [<>](https://github.com/d3/d3-force/blob/master/src/radial.js "Source")
 
-If *y* is specified, sets the *y*-coordinate of the circle center to the specified number and returns this force. If *y* is not specified, returns the current *y*-coordinate of the center, which defaults to zero.
+If *y* is specified, sets the *y*-coordinate accessor to the specified number or function, re-evaluates the *y*-accessor for each node, and returns this force. If *y* is not specified, returns the current *y*-accessor, which defaults to:
+
+```js
+function y() {
+  return 0;
+}
+```
+
+The *y*-accessor is invoked for each [node](#simulation_nodes) in the simulation, being passed the *node* and its zero-based *index*. The resulting number is then stored internally, such that the target *y*-coordinate of each node is only recomputed when the force is initialized or when this method is called with a new *y*, and not on every application of the force.
+