[docs] Document max_request_body_size setting of HTTPServerSettings (#…

…9014)

Document `max_request_body_size` setting of `HTTPServerSettings`:
https://github.com/open-telemetry/opentelemetry-collector/blob/daef971a0b3087ed76ebe9fd4793877b41b65306/config/confighttp/confighttp.go#L241

Signed-off-by: Andreas Gerstmayr <agerstmayr@redhat.com>

config/confighttp/README.md

@@ -69,6 +69,7 @@ will not be enabled.
   header, allowing clients to cache the response to CORS preflight requests. If
   not set, browsers use a default of 5 seconds.
 - `endpoint`: Valid value syntax available [here](https://github.com/grpc/grpc/blob/master/doc/naming.md)
+- `max_request_body_size`: configures the maximum allowed body size in bytes for a single request. Default: `0` (no restriction)
 - [`tls`](../configtls/README.md)
 - [`auth`](../configauth/README.md)