[HttpFoundation] Documented public properties

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9670
| License       | MIT
| Doc PR        | -

`$request` and `$query` parameter bags could be hard to get for a newcommer, who's used to `$_POST` and `$_GET` variables. For consistency I documented all public properties.