stale-bot: Make it's messages more instructive
stale-bot: Make it's messages more instructive

As somewhat discussed at: https://git.io/JJkRa, make the bot's message a
bit more instructive, and clearer as for what it will and won't do, so
random contributors will have a better idea what this is about.

Also: Split stale issues' and PRs' messages because each should
suggest different actions to do.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs