boards: st: stm32f429i_disc1: doc: re-work flashing section

The patch modifies the "Programming and Debugging" and "Flashing" sections
to make the documentation clearer and consistent with what is reported for
other ST boards.

Suggested-by: Abderrahmane Jarmouni <git@jarmouni.me>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>