Add docs about suppressed function cast warning

Change-Id: I80f38908c96907cb641034d290c500e7fa2c3e69
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/610656
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed