Fix documentation for max_request_body_size (#11074)

#### Description

Fix documentation for max_request_body_size 

#### Link to tracking issue
Fixes #11066

Author: ntkme

config/confighttp/README.md

@@ -78,7 +78,7 @@ 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)
+- `max_request_body_size`: configures the maximum allowed body size in bytes for a single request. Default: `20971520` (20MiB)
 - `compression_algorithms`: configures the list of compression algorithms the server can accept. Default: ["", "gzip", "zstd", "zlib", "snappy", "deflate"]
 - [`tls`](../configtls/README.md)
 - [`auth`](../configauth/README.md)

receiver/otlpreceiver/config.md

@@ -76,7 +76,7 @@ Config defines configuration for OTLP receiver.
 | endpoint              | string                                                    | localhost:4318 | Endpoint configures the listening address for the server.                                                                               |
 | tls                   | [configtls-TLSServerSetting](#configtls-tlsserversetting) | <no value>   | TLSSetting struct exposes TLS client configuration.                                                                                     |
 | cors                  | [confighttp-CORSConfig](#confighttp-corsconfig)           | <no value>   | CORSConfig configures a receiver for HTTP cross-origin resource sharing (CORS).                                                       |
-| max_request_body_size | int                                                       | 0            | MaxRequestBodySize configures the maximum allowed body size in bytes for a single request. The default `0` means there's no restriction |
+| max_request_body_size | int                                                       | 20971520     | MaxRequestBodySize configures the maximum allowed body size in bytes for a single request. The default `20971520` means 20MiB           |
 
 ### confighttp-CORSConfig