Improve `InvalidTransaction::AncientBirthBlock` doc comment (#7743)

* Improve `InvalidTransaction::AncientBirthBlock` doc comment

Update

Remove stray parentheses

Tabs not spaces

Improve

whoops

* Update primitives/runtime/src/transaction_validity.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Possible causes section; Expand example

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>