Document point addition internal implementation

[vlopes11]

what are x_3 and y_3?

Originally posted by @moCello in https://github.com/dusk-network/plonk/pull/700#discussion_r946992589

[ashWhiteHat]

I think x_3 and x_3 are coordinates of added point. component_add_point constrains a and b JubjubAffine points addition a + b = c.

a: (x_1, y_1) b: (x_2, y_2) c (a + b): (x_3, y_3)

In dusk (zkcrypto) Jubjub implementation, the coordinates are expressed as u (x) and v (y). We should align document notation with either of them.