doc: prevent writing "topic:" to commit message

doc: prevent writing "topic:" to commit message

First time contributors surprisingly often write "topic:"
instead of "the main part or subsystem the patch touches" as requested
in patchguide.html

To prevent them doing so:
Use longer self-explaing "specify touched area" instead of "topic"
Give some examples in addition to recommended looking at "git log"

Change-Id: I1c307b460d7a79ba3c9918af8dbc9e9f827e1fb9
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6683
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: Paul Fertser <fercerpav@gmail.com>