Add documentation for updating the issue triage oncall

Summary:
This Diff simply adds some docs on how the secret must be formatted, and removes a print that would expose the triager IDs is anyone would look at the logs (not a big deal given that the ids are public on Discord, though).

## Changelog:
[Internal] -

Reviewed By: cortinico

Differential Revision: D79556988

fbshipit-source-id: 23d6e72141dff4e91242cc1d9f5b95ebaf5ca858