Fix broken docstring rendering in Linear and RNN - #3890

Proposed changes

Two small docstring rendering fixes, no behavior change:

1. Linear: the docstring had a duplicated where: line (a bare where: immediately followed by where :math:\W` has shape ...`), so it rendered "where:" twice. Removed the stray line.
2. RNN: the nonlinearity arg wrote func:\tanh`without the leading colon, so Sphinx rendered it literally instead of as a:func:cross-reference. Fixed to:func:`tanh``.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

(docstring-only, no tests needed)

small disclaimer: i'm a freshman contributor and i used Claude Code to help me sweep for docstring issues, but i read both docstrings myself to confirm these actually render wrong before opening this. let me know if anything looks off :)