message: docs(forms): add diff between add and set control
  PR Close
diff: @@ -1457,7 +1457,9 @@ export class FormGroup extends AbstractControl {
  /**
  * Add a control to this group.
  *
  - * This method also updates the value and validity of the control.
  + * If a control with a given name already exists, it would *not* be replaced with a new one.
  + * If you want to replace an existing control, use the {@link FormGroup#setControl setControl}
  + * method instead. This method also updates the value and validity of the control.
  *
  * @param name The control name to add to the collection
  * @param control Provides the control for the given name
  @@ -1495,6 +1497,8 @@ export class FormGroup extends AbstractControl {
  /**
  * Replace an existing control.
  *
  + * If a control with a given name does not exist in this `FormGroup`, it will be added.
  + *
  * @param name The control name to replace in the collection
  * @param control Provides the control for the given name
  * @param options Specifies whether this FormGroup instance should emit events after an
  