Add a descriptive comment about the MAXSKINS macro

src/doomdef.h

@@ -223,8 +223,13 @@ extern char logfilename[1024];
 #define MAXPLAYERNAME 21
 #define PLAYERSMASK (MAXPLAYERS-1)
 
+// Don't make MAXSKINS higher than 256, since skin numbers are used with an
+// UINT8 in various parts of the codebase. If you do anyway, the data type
+// of those variables will have to be changed into at least an UINT16.
+// This change must affect code such as demo recording and playback,
+// and the structure of some networking packets and commands.
 #define MAXSKINS 256
-#define MAXCHARACTERSLOTS (MAXSKINS * 3)
+#define MAXCHARACTERSLOTS (MAXSKINS * 3) // Should be higher than MAXSKINS.
 
 #define COLORRAMPSIZE 16
 #define MAXCOLORNAME 32