Document post-prime and pack steps

[mr-cal]

What needs to get done

Document what happens during post-prime and pack, such as the generating metadata and copying assets.

Why it needs to get done

Snapcraft makes many modifications to the prime directory before packing. These are not documented and it not clear to users what Snapcraft is doing prior to calling snap pack.

source: https://forum.snapcraft.io/t/snap-is-unusable-due-to-bad-permissions/41498/3

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3266.

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