Docs improvements about around sigv4

[iwysiu]

We have 2 older but still applicable support escalations about there not being enough detail about sigv4 enablement in the the docs:

• https://github.com/grafana/support-escalations/issues/9929
  • Requests more info about "Workspace IAM Role" option under SigV4 auth in Prometheus datasource.
• https://github.com/grafana/support-escalations/issues/11315
  • Requests a note about mentioning that sigv4_auth_enabled needs to be set for sigv4

Two of these involve them not catching that they have to update the grafana config, and while at least the first one is technically in the linked aws docs, we should look into calling them out more obviously (maybe in the migration section?)