Document all exponents in the FieldElement repr

Part of agl#14.

edwards25519/edwards25519.go

@@ -10,10 +10,10 @@ package edwards25519
 // This code is a port of the public domain, "ref10" implementation of ed25519
 // from SUPERCOP.
 
-// FieldElement represents an element of the field GF(2^255 - 19).  An element
+// FieldElement represents an element of the field GF(2^255 - 19). An element
 // t, entries t[0]...t[9], represents the integer t[0]+2^26 t[1]+2^51 t[2]+2^77
-// t[3]+2^102 t[4]+...+2^230 t[9].  Bounds on each t[i] vary depending on
-// context.
+// t[3]+2^102 t[4]+2^128 t[5]+2^153 t[6]+2^179 t[7]+2^204 t[8]+2^230 t[9].  
+// Bounds on each t[i] vary depending on context.
 type FieldElement [10]int32
 
 var zero FieldElement