[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.
Merge branch '2.3' into 2.4

* 2.3:
  fixed CS
  allow the TextAreaFormField to be used with valid/invalid HTML
  Create security.id.xlf
  [DependencyInjection] Remove unneeded file
  [Twig] removed obsolete conditions on Twig versions
  added lines to exceptions for the trans and transchoice tags
  [Form] Make FormInterface::add docblock more explicit
  [Security] Add zh_CN translations
  [Validator][Translation]update zh_CN translations
  [Validator] Minor fix in XmlLoader
Merge branch '2.4'

* 2.4:
  fixed CS
  allow the TextAreaFormField to be used with valid/invalid HTML
  Create security.id.xlf
  [DependencyInjection] Remove unneeded file
  [Twig] removed obsolete conditions on Twig versions
  added lines to exceptions for the trans and transchoice tags
  [FrameworkBundle] Merged multiple line input into one line to fix the tests.
  [Form] Make FormInterface::add docblock more explicit
  [Security] Add zh_CN translations
  Routing condition bugfix
  [Validator][Translation]update zh_CN translations
  [Validator] Minor fix in XmlLoader