[Form] Make FormInterface::add docblock more explicit

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

When replacing a child form from within a form event, it's not obvious that you have to use the 'add' method. Though it's minor, this will save somebody a google search.