Add a performance section to TPU docs to address FAQ  (#5445)
Add a performance section to TPU docs to address FAQ  (#5445)

* header

* update docs

* punctuation

* adding another note

* some more notes

* Update docs/source/tpu.rst

Co-authored-by: Rohit Gupta <cb3c4569854e7764e66b4cfd98eab60a0e5fb067@gmail.com>

* punctuation

Co-authored-by: Lezwon Castelino <04d7a8e5ac110415dc5097095d306c30cf4d97d9@gmail.com>
Co-authored-by: Rohit Gupta <cb3c4569854e7764e66b4cfd98eab60a0e5fb067@gmail.com>
Co-authored-by: chaton <5f50a84c1fa3bcff146405017f36aec1a10a9e38@grid.ai>
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning