Document PROTOCOL_VERSION

Marlin/Version.h

   #define REQUIRED_CONFIGURATION_ADV_H_VERSION 010100
 
   /**
-   * @todo: Missing documentation block
+   * The protocol for communication to the host. Protocol indicates communication
+   * standards such as the use of ASCII, "echo:" and "error:" line prefixes, etc.
+   * (Other behaviors are given by the firmware version and capabilities report.)
    */
   #define PROTOCOL_VERSION "1.0"