Add: support filtering content entries for patchpacks (#9541)

This changes nothing for us, but allows patchpacks to add the
right pieces of code to start filtering content entries on patchpack
only entries.

src/network/core/tcp_content.h

@@ -26,7 +26,11 @@ protected:
 	/**
 	 * Client requesting a list of content info:
 	 *  byte    type
-	 *  uint32  openttd version
+	 *  uint32  openttd version (or 0xFFFFFFFF if using a list)
+	 * Only if the above value is 0xFFFFFFFF:
+	 *  uint8   count
+	 *  string  branch-name ("vanilla" for upstream OpenTTD)
+	 *  string  release version (like "12.0")
 	 * @param p The packet that was just received.
 	 * @return True upon success, otherwise false.
 	 */