Board: disco_l475_iot1: Fix PWM assignment

Most people would expect green_pwm_1 to go to LD1 on the dev board.
However it is routed to CN1 pin 2, since routing it to LD1 would
interfere with SPI SCK on the Arduino connector. This PR adds a
comment to tell users that LD1 won't light up as expected and to
fix the comments stating which PWM goes to which pin.

Also updated the documentation to note this.

Signed-off-by: Glenn Andrews <glenn.andrews.42@gmail.com>