Add comment for device path protocol.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@6175 6f19259b-4bc3-4df7-8a09-765794883524

MdePkg/Include/Protocol/DevicePath.h

@@ -40,10 +40,30 @@
 
 #pragma pack(1)
 
+/**
+  This protocol can be used on any device handle to obtain generic path/location 
+  information concerning the physical device or logical device. If the handle does 
+  not logically map to a physical device, the handle may not necessarily support 
+  the device path protocol. The device path describes the location of the device 
+  the handle is for. The size of the Device Path can be determined from the structures 
+  that make up the Device Path.
+**/
 typedef struct {
-  UINT8 Type;
-  UINT8 SubType;
-  UINT8 Length[2];
+  UINT8 Type;       ///< 0x01 - Hardware Device Path
+                    ///< 0x02 – ACPI Device Path
+                    ///< 0x03 – Messaging Device Path
+                    ///< 0x04 – Media Device Path
+                    ///< 0x05 – BIOS Boot Specification Device Path
+                    ///< 0x7F – End of Hardware Device Path
+                    
+  UINT8 SubType;    ///< Varies by Type
+                    ///< 0xFF – End Entire Device Path, or
+                    ///< 0x01 – End This Instance of a Device Path and start a new
+                    ///< Device Path
+                    
+  UINT8 Length[2];  ///< Specific Device Path data. Type and Sub-Type define
+                    ///< type of data. Size of data is included in Length.
+                    
 } EFI_DEVICE_PATH_PROTOCOL;
 
 ///