docs: clarify Bisection bound differentiation

[Haihan-Jiang]

Addresses #472.

What

• Clarify in the root-finding docs that Bisection differentiates roots with respect to arguments passed to run, not with respect to the bracketing bounds.
• Note that lower and upper are solver hyperparameters and should have their gradients stopped when they are computed from differentiable JAX values only to define the search interval.
• Mirror that guidance in the Bisection docstring so generated API docs include it.

Testing

• git diff --check
• python3.12 -m compileall -q jaxopt/_src/bisection.py
• Temporary venv smoke test importing Bisection and checking the updated docstring
• Temporary venv smoke test running the root-finding differentiation example with jax.grad

The full Sphinx docs build was not run locally because the docs requirements install the full documentation stack, including TensorFlow/Flax.