internal/cache: document 'go clean -fuzzcache' in README
internal/cache: document 'go clean -fuzzcache' in README

Fixes: #48900

Change-Id: I9235441886ed7cbdfdcbd283480f52d9216c3ea2
Reviewed-on: https://go-review.googlesource.com/c/go/+/357233
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

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