Also update some snapshot agent docs
* Enforce correct permissions when registering a check
Previously we had attempted to enforce service:write for a check associated with a service instead of node:write on the agent but due to how we decoded the health check from the request it would never do it properly. This commit fixes that.
* Update website/source/docs/commands/snapshot/agent.html.markdown.erb
Co-Authored-By: mkeeler <mkeeler@users.noreply.github.com>