doc/go1.17: mention new vet checks sigchanyzer and stdmethods.
doc/go1.17: mention new vet checks sigchanyzer and stdmethods.

These vet checks were added in CL 299532 and CL 321389.

Also adds a TODO for buildtags.

Change-Id: I516dc77729f6d2dc147318260fe452831b115dfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/322769
Trust: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
Run-TryBot: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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