[docs] [issue41411] Improve and consolidate f-strings docs

Eric V. Smith <eric@trueblade.com> added the comment: I think this is an excellent idea. The main f-string docs being in a section titled "Lexical Analysis" never seemed very user-friendly. ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue41411> _______________________________________