[FEATURE] Documentation on reference direction of branch and branch3

[TonyXiang8787]

The field p_from, and p_to of the branch, is not clear which direction of the power is. While it is always from node to the branch, it is good to document them explicitly. Also includes branch3.

Edit:
add two bullet points for branch/branch3 on https://power-grid-model.readthedocs.io/en/stable/user_manual/data-model.html#reference-direction