doc: function help for MP_SIZE macro

Includes warning to use 4K stack in any task calling into MPARITH.LIB.

Lib/Rabbit4000/Crypto/MPARITH.LIB

@@ -62,12 +62,25 @@ END DESCRIPTION ***************************************************************/
 #ifndef MPARITH_H
 #define MPARITH_H
 
+/* START FUNCTION DESCRIPTION ********************************************
+MP_SIZE                                                      <MPARITH.LIB>
+
+SYNTAX:	#define MP_SIZE 258
+
+DESCRIPTION:	This MACRO controls the maximum key size supported by
+               MPARITH.LIB.  Defined as the number of bytes in modulus,
+               plus 2 for temporary 16-bit carry out.  Must be a multiple
+               of 4, plus 2.
+
+               130 : 1024-bit keys
+               258 : 2048-bit keys (default value)
+               514 : 4096-bit keys
+
+               An MP_SIZE of 514 increases stack usage in this library so
+               all uC/OS-II tasks calling into the library (including tasks
+               using TLS/SSL) should use a 4KB stack.
+END DESCRIPTION **********************************************************/
 #ifndef MP_SIZE
-	// Number of bytes in modulus, plus 2 for temporary 16-bit carry out
-   // Make this a multiple of 4, plus 2.  The default, 258, specifies
-	// 2048-bit key lengths.  This is the maximum key size supported;
-	// all shorter lengths are automatically supported.
-   // Use a value of 130 for 1024-bit keys, or 514 for 4096-bit keys.
 	#define MP_SIZE 258
 #endif