design/draft-fuzzing.md: clarify open issues
design/draft-fuzzing.md: clarify open issues

Change-Id: I29425b97c3118f4a5697b047bcbd9301d59cb241
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/295030
Trust: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Run-TryBot: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

golang/proposal,golang/proposal