updated Creating a Custom Form Field doc ngControl section (#9855)

* Creating a custom form field docs -- updated ngControl section to reflect errors injecting NgControl

If the component already implements ControlValueAccessor, injecting NgControl in the constructor (as described in the docs) may fail. Added an alternate way.

* Update creating-a-custom-form-field-control.md

* added word "provider" for clarity

guides/creating-a-custom-form-field-control.md

@@ -157,23 +157,43 @@ private _placeholder: string;
 
 #### `ngControl`
 
-This property allows the form field control to specify the `@angular/forms` control that is bound to
-this component. Since we haven't set up our component to act as a `ControlValueAccessor`, we'll just
-set this to `null` in our component. In any real component, you would probably want to implement
-`ControlValueAccessor` so that your component can work with `formControl` and `ngModel`.
+This property allows the form field control to specify the `@angular/forms` control that is bound to this component. Since we haven't set up our component to act as a `ControlValueAccessor`, we'll just set this to `null` in our component. 
 
 ```ts
-ngControl = null;
+ngControl: NgControl = null;
 ```
 
-If you did implement `ControlValueAccessor`, you could simply inject the `NgControl` and make it
-publicly available. (For additional information about `ControlValueAccessor` see the
-[API docs](https://angular.io/api/forms/ControlValueAccessor).)
+It is likely you will want to implement `ControlValueAccessor` so that your component can work with `formControl` and `ngModel`. If you do implement `ControlValueAccessor` you will need to get a reference to the `NgControl` associated with your control and make it publicly available. 
+
+The easy way is to add it as a public property to your constructor and let dependency injection handle it:
 
 ```ts
-constructor(..., @Optional() @Self() public ngControl: NgControl) { ... }
+constructor(
+  ..., 
+  @Optional() @Self() public ngControl: NgControl,
+  ...,
+) { }
 ```
 
+Note that if your component implements `ControlValueAccessor`, it may already be set up to provide `NG_VALUE_ACCESSOR` (in the `providers` part of the component's decorator, or possibly in a module declaration). If so you may get a *cannot instantiate cyclic dependency* error. 
+
+To resolve this, remove the `NG_VALUE_ACCESSOR` provider and instead set the value accessor directly:
+
+```ts
+constructor(
+  ..., 
+  @Optional() @Self() public ngControl: NgControl,
+  ...,
+) {
+  // Setting the value accessor directly (instead of using
+  // the providers) to avoid running into a circular import.
+  if (this.ngControl != null) { this.ngControl.valueAccessor = this; }
+}
+```
+
+For additional information about `ControlValueAccessor` see the [API docs](https://angular.io/api/forms/ControlValueAccessor).
+
+
 #### `focused`
 
 This property indicates whether or not the form field control should be considered to be in a