protocol.h: Make enums in GetDataMsg concrete values

This concretizes the numbers and adds a comment to make it clear that
these numbers are fixed by the protocol, and may avoid people forgetting
to claim numbers in the future (e.g. issue #8500).

Also gets rid of a weird unused `MSG_TYPE_MAX` in the middle of the
enumeration (thanks @paveljanik for noticing).

(cherry picked from commit bitcoin/bitcoin@1df3111)

src/protocol.h

@@ -116,15 +116,18 @@ class CAddress : public CService
     unsigned int nTime;
 };
 
-/** getdata message types */
+/** getdata / inv message types.
+ * These numbers are defined by the protocol. When adding a new value, be sure
+ * to mention it in the respective ZIP, as well as checking for collisions with
+ * BIPs we might want to backport.
+ */
 enum GetDataMsg
 {
     UNDEFINED = 0,
-    MSG_TX,
-    MSG_BLOCK,
-    MSG_TYPE_MAX = MSG_BLOCK,
+    MSG_TX = 1,
+    MSG_BLOCK = 2,
     // The following can only occur in getdata. Invs always use TX or BLOCK.
-    MSG_FILTERED_BLOCK,
+    MSG_FILTERED_BLOCK = 3,  //!< Defined in BIP37
 };
 
 /** inv message data */