doc/go1.18: mention new cmd/go fuzzing flags
doc/go1.18: mention new cmd/go fuzzing flags

For #47694

Change-Id: I00da9bd39700e938ec492daa71aba2035d911a06
Reviewed-on: https://go-review.googlesource.com/c/go/+/374354
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

nawawi/go,cloudflare/go,cloudflare/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go