DOC Clarify primal and dual formulations in Ridge solvers - #34899

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Closing my duplicate (#34893) in favour of this one — you had the assignment, and the user-guide half here is the more complete fix. Two things from my version that might be worth folding in:

1. Writing the dual system out explicitly in the docstrings. The RidgeCV Notes section in the same file already uses this notation (w = H^-1 X' y = X' c where c = G^-1 y), so c stays consistent with what's there:

... solves (X X' + alpha*I) c = y for the dual coefficients c, and obtains coef_ from X' c

2. The coefficient orientation. X' c comes out as (n_features, n_targets), while coef_ is (n_targets, n_features) — hence the trailing .T at _ridge.py:763. Worth stating, since a reader following the math otherwise lands on the transposed shape. Checked on a dual-branch fit with n_samples=50, n_features=200, n_targets=3: c is (50, 3), X' c is (200, 3), and coef_ is (3, 200).

Both are docstring-only, so take or leave them as you prefer.

(Edited to fix transposed shape figures in point 2.)