docs: describe storage limitations in hybrid and Konnect for ACME plugin

[lena-larionova]

Summary

Updating the ACME plugin's config.storage parameter description to:

• Mention that shm is not supported in hybrid, and kong storage is not supported in Konnect.
• Remove the line about supported backends, as those as listed in the one_of field in the schema.

Resolves the last piece of https://konghq.atlassian.net/browse/DOCU-2910. Issue recently came up with a user, see link to slack convo in ticket.

Checklist

• [ N/ A ] The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - ACME plugin storage types docs.konghq.com#7996