Fix searchsorted function signature in docs (#120086)
Side should be optional string, to match definition in native_functions: https://github.com/pytorch/pytorch/blob/fbe8e0f92d4c21d0ba192b9101097c575690a3f1/aten/src/ATen/native/native_functions.yaml#L11246
Fixes https://github.com/pytorch/pytorch/issues/119999
Test plan: https://docs-preview.pytorch.org/pytorch/pytorch/120086/generated/torch.searchsorted.html#torch-searchsorted
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120086
Approved by: https://github.com/lezcano
