Can't upload sketches longer than 122880 bytes on atmega1284.

http://code.google.com/p/optiboot/issues/detail?id=57
Correct NRWWSTART for 1284; byte vs word address confusion.
Add explanatory comments!

optiboot/bootloaders/optiboot/optiboot.c

@@ -259,6 +259,21 @@ void uartDelay() __attribute__ ((naked));
 #endif
 void appStart() __attribute__ ((naked));
 
+/*
+ * NRWW memory
+ * Addresses below NRWW (Non-Read-While-Write) can be programmed while
+ * continuing to run code from flash, slightly speeding up programming
+ * time.  Beware that Atmel data sheets specify this as a WORD address,
+ * while optiboot will be comparing against a 16-bit byte address.  This
+ * means that on a part with 128kB of memory, the upper part of the lower
+ * 64k will get NRWW processing as well, even though it doesn't need it.
+ * That's OK.  In fact, you can disable the overlapping processing for
+ * a part entirely by setting NRWWSTART to zero.  This reduces code
+ * space a bit, at the expense of being slightly slower, overall.
+ *
+ * RAMSTART should be self-explanatory.  It's bigger on parts with a
+ * lot of peripheral registers.
+ */
 #if defined(__AVR_ATmega168__)
 #define RAMSTART (0x100)
 #define NRWWSTART (0x3800)
@@ -270,7 +285,7 @@ void appStart() __attribute__ ((naked));
 #define NRWWSTART (0xE000)
 #elif defined (__AVR_ATmega1284P__)
 #define RAMSTART (0x100)
-#define NRWWSTART (0xF000)
+#define NRWWSTART (0xE000)
 #elif defined(__AVR_ATtiny84__)
 #define RAMSTART (0x100)
 #define NRWWSTART (0x0000)