Snapcraft: Describe key-id limitations

[akcano]

Update the following guide:

Package repositories (key-id)

Background

The documentation doesn't mention that multiple GPG keys cannot be specified for a single APT repo, though adding multiple repo entries in the Snapcraft YAML file with the same URL and different keys seems to work.

Prerequisites

Familiarity with Snapcraft or snaps is needed to understand the content and test any conjectures. The documentation is Discourse-hosted, so some experience with Discourse is also a plus.