Document constants and FieldElement

[benma]

ed25519/edwards25519/edwards25519.go

Lines 13 to 16 in 278e1ec

	// 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.

Please expand the "..." and document where does exponents (26, 51, ...) come from.

Please document a bit what the constants mean here:
https://github.com/agl/ed25519/blob/278e1ec8e8a6e017cd07577924d6766039146ced/edwards25519/const.go