docs: clarify alias usage for x parameter in vector_norm function (#136921)

- Added a note in the documentation specifying that the `input` parameter can be used as an alias for `x`.

Fixes #136560

Pull Request resolved: https://github.com/pytorch/pytorch/pull/136921
Approved by: https://github.com/ezyang

Co-authored-by: Edward Z. Yang <ezyang@meta.com>

torch/linalg/__init__.py

@@ -1440,7 +1440,8 @@ but it is faster in some cases.
 
 Args:
     x (Tensor): tensor, flattened by default, but this behavior can be
-        controlled using :attr:`dim`.
+        controlled using :attr:`dim`.  (Note: the keyword argument
+        `input` can also be used as an alias for `x`.)
     ord (int, float, inf, -inf, 'fro', 'nuc', optional): order of norm. Default: `2`
     dim (int, Tuple[int], optional): dimensions over which to compute
         the norm. See above for the behavior when :attr:`dim`\ `= None`.