argon2: update RFC 9106 parameter recommendations

Update the IDKey example to use RFC 9106's first recommended Argon2id
option and clarify the relationship between the first and second
recommended options.

Fixes golang/go#79823

Change-Id: If44a86a5a8112b02fab97b4d5e325fe91d6049ee
GitHub-Last-Rev: a764ae65875f12d7422865c4e83f345a8a4f246e
GitHub-Pull-Request: golang/crypto#357
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/789020
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>