[HttpFoundation] Add doc for Request::getContent() method
feature #3610 [HttpFoundation] Add doc for Request::getContent() method (bicpi)

This PR was merged into the 2.3 branch.

Discussion
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[HttpFoundation] Add doc for Request::getContent() method

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #3609

Commits
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a61c137 [HttpFoundation] Add doc for Request::getContent() method