[docs] Merge note block in `torch.lu` documentation (#63156)

[docs] Merge note block in `torch.lu` documentation (#63156)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63156

**Summary**
This commit merges the four successive `Note` blocks that appear in the
documentation for `torch.lu`. Each one only has one line in it, so all
of them have been merged into one block with a bulleted list that
contains the original items.

**Test Plan**
Continuous integration.

*Before*
<img width="888" alt="Captura de Pantalla 2021-08-12 a la(s) 10 48 39 a  m" src="https://user-images.githubusercontent.com/4392003/129244443-b7d1594e-8833-4c20-a911-e1bf7ca88a8d.png">

*After*
<img width="932" alt="Captura de Pantalla 2021-08-12 a la(s) 10 48 46 a  m" src="https://user-images.githubusercontent.com/4392003/129244462-1f39dcdb-90e0-4fd9-a95f-343b0b6be1f1.png">

**Fixes**
This commit fixes #62339.

Test Plan: Imported from OSS

Reviewed By: navahgar, pbelevich

Differential Revision: D30292633

Pulled By: SplitInfinity

fbshipit-source-id: cb9071165629bfe7316b1d2fe952e4354c75d48f