bpo-39096: Improve description of 'e', 'f' and 'g' presentation types (GH-23537)...

bpo-39096: Improve description of 'e', 'f' and 'g' presentation types (GH-23537) (GH-23550)

* Improve description of 'e', 'f' and 'g' presentation types

* Drop the 'E' from Scientific 'E' notation; remove >= 0 qualifications

* Fix false statement that the alternate form is valid for Decimal

* Nitpick: remove the Harvard/Oxford comma

* Add note that the decimal point is also removed if no digits follow it, except in alternate form
(cherry picked from commit c642374b3ef72f6f300616f07aea2a3f9ed83e51)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
Co-authored-by: Mark Dickinson <mdickinson@enthought.com>