in_udp: add recv_buffer parameter (fluent#517)

This parameter was introduced at fluent/fluentd@8625d40

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

input/udp.md

@@ -135,6 +135,14 @@ The maximum number of bytes for the message.
 
 Removes the newline from the end of the incoming payload.
 
+### `receive_buffer_size`
+
+| type | default | version |
+| :--- | :--- | :--- |
+| size | nil | 1.0.1 |
+
+The max size of socket receive buffer. This is used in `SO_RCVBUF` socket option.
+
 ### `<parse>` Section
 
 | required | multi | version |