docs(forms): Added FormControl initialization information (#11948)

modules/@angular/forms/src/model.ts

@@ -625,6 +625,8 @@ export abstract class AbstractControl {
  *
  * You can also initialize the control with a form state object on instantiation,
  * which includes both the value and whether or not the control is disabled.
+ * You can't use the value key without the disabled key; both are required
+ * to use this way of initialization.
  *
  * ```ts
  * const ctrl = new FormControl({value: 'n/a', disabled: true});