[HttpClient] fix documentation of retry_failed

[Amunak]

The documentation for retry_failed / RetryableHttpClient implies that this feature is enabled by default:

Sometimes, requests fail because of network issues or temporary server errors. Symfony's HttpClient allows to retry failed requests automatically using the retry_failed option.

By default, failed requests are retried up to 3 times, [...]

What does the "by default" here refer to? Because by default it's certainly not enabled. It should be clarified.

The framework configuration hints more that this isn't the case:

By enabling the optional retry_failed configuration, the HTTP client service will automatically retry failed HTTP requests.

I feel like the documentation here is in general kinda confusing, since it's split between two places, worded differently and there is some information in one place, some in the other, and some in both. The result is confusing and to me (at least the first instance) implies that it's enabled by default.