Improve LSTM documentation for proj_size > 0 (#65102)

Improve LSTM documentation for proj_size > 0 (#65102)

Summary:
Fixes https://github.com/pytorch/pytorch/issues/65053. Although the documentation states that:

https://github.com/pytorch/pytorch/blob/fe0f9d1dafb9791cb08635636a01128850d17538/torch/nn/modules/rnn.py#L500-L506

It seems that the definition of `weight_ih_l[k]` could be improved by specifying what happens when `k > 0` and `proj_size > 0`. As `proj_size` is only used in LSTM, no changes are needed for the other RNNs.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/65102

Reviewed By: supriyar

Differential Revision: D30975781

Pulled By: jbschlosser

fbshipit-source-id: 12df06e5e6a8d5de0ad10fb15e33c3e6311c11d3