DOC correct Xy shape in lars_path and lars_path_gram

[yuanx749]

Reference Issues/PRs

Ran into the same issue as #27887 when using lars_path.

What does this implement/fix? Explain your changes.

After going through the source code of _lars_path_solver, I think these functions only work for single-target y, therefore suggest the changes in docstrings.

Any other comments?

Code to reproduce errors.

from sklearn.datasets import make_regression
from sklearn.linear_model import lars_path, lars_path_gram
X, y = make_regression(n_features=10, n_targets=2, random_state=0)
Xy = np.dot(X.T, y)
Gram = np.dot(X.T, X)
lars_path(X, y, Xy=Xy, Gram=Gram)  # ValueError
lars_path_gram(Xy, Gram=Gram, n_samples=X.shape[0])  # ValueError

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[glemaitre]

LGTM