[Form] fix and unify phpdoc

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| license?      | MIT

Fixes GuesserInterface that must return ValueGuess instead of abstract Guess.
Unify array typehints that was not done consistently.