net: ena: Add more documentation for RX copybreak

This patch contains more details about the functionality
of RX copybreak.
Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David Arinzon <darinzon@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/device_drivers/ethernet/amazon/ena.rst

@@ -211,10 +211,16 @@ Documentation/networking/net_dim.rst
 
 RX copybreak
 ============
+
 The rx_copybreak is initialized by default to ENA_DEFAULT_RX_COPYBREAK
 and can be configured by the ETHTOOL_STUNABLE command of the
 SIOCETHTOOL ioctl.
 
+This option controls the maximum packet length for which the RX
+descriptor it was received on would be recycled. When a packet smaller
+than RX copybreak bytes is received, it is copied into a new memory
+buffer and the RX descriptor is returned to HW.
+
 Statistics
 ==========