Logging Conventions contain non-existent log level

[progmem]

Looking at the documentation for v3.5, v3.6, and the draft v3.7, there's a log level being described that doesn't exist within zap:

• Notice: Normal, but important (uncommon) log information.
  • Examples:
    • Add a new node into the cluster
    • Add a new user into auth subsystem

This commit shows a switchover from capnslog to zap; it appears during this switchover that the documentation itself wasn't adjusted to compensate.