Clarify that each platform entry is a 1:1 mapping to output charm files #1963
Description
What needs to get done
Clarify in the charmcraft.yaml schema that each entry in platforms will result in a separate charm file on packing.
Why it needs to get done
We got some questions/confusion about platforms - we should note that each entry specifically becomes its own charm file.
https://matrix.to/#/!LhFxJIPEcCacgdMghH:ubuntu.com/$XzGzD_5XLPXzfN2VqSMuoj4-t4C5MH-2nxv7Fh9ZRsY