Transaction Pool docs (#9056)

* Add transaction pool docs.

* Extra docs.

* Apply suggestions from code review

Co-authored-by: Pierre Krieger <[email protected]>

* Expand on some review comments.

* Update README.md

Fixed typos / spellings

Co-authored-by: Pierre Krieger <[email protected]>
Co-authored-by: Squirrel <[email protected]>