Doc improvement on Paillier crypto system

[vadym-f]

To keep the documentation complete
(avoiding the need to get Paillier encrypt/decrypt specification),
one would include L(x) function definition:

if
N \not | (x-1) return wrong_ciphertext
else
return \frac {x-1} {N}

Also, for any a \in Z_N: (N+1)^a = 1 + aN \pmod{N^2}