Default MAC address generation is not random and should be stated in the documentation.

Signed-off-by: Gianluca Borello <g.borello@gmail.com>

daemon/networkdriver/bridge/driver.go

@@ -608,7 +608,7 @@ func Allocate(id, requestedMac, requestedIP, requestedIPv6 string) (*network.Set
 		return nil, err
 	}
 
-	// If no explicit mac address was given, generate a random one.
+	// If no explicit mac address was given, generate one from the IP address.
 	if mac, err = net.ParseMAC(requestedMac); err != nil {
 		mac = generateMacAddr(ip)
 	}

docs/sources/reference/run.md

@@ -216,9 +216,9 @@ networking. In cases like this, you would perform I/O through files or
 Your container will use the same DNS servers as the host by default, but
 you can override this with `--dns`.
 
-By default a random MAC is generated. You can set the container's MAC address
-explicitly by providing a MAC via the `--mac-address` parameter (format:
-`12:34:56:78:9a:bc`).
+By default, the MAC address is generated using the IP address allocated to the
+container. You can set the container's MAC address explicitly by providing a
+MAC address via the `--mac-address` parameter (format:`12:34:56:78:9a:bc`).
 
 Supported networking modes are: