improvements to torch.gradient docs (#98824)
Fixes #98693
Clarified docs for `torch.gradient` on `h_l` and how the gradient is computed. For the mathematical equations, I followed this reference: https://www.dam.brown.edu/people/alcyew/handouts/numdiff.pdf.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98824
Approved by: https://github.com/ngimel, https://github.com/kit1980
