tcl/target: stm32[fl]4x: document the settings for trace

tcl/target: stm32[fl]4x: document the settings for trace

While reviewing on gerrit the change
	https://review.openocd.org/6932/
it get clear that the missing documentation on stm32f4x's code
was triggering errors in the new change.

OpenOCD is currently unable to read traces, but these can be
hopefully be read with some other tool.

Document the settings for enabling trace on stm32[fl]4x.

Change-Id: Ibae77a53de16375d3d500e728678740095547009
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6945
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>