Add content to section TPM-Backed FDE

[jibel]

Adding a release note

Feel free to delete this template if you're making a minor or unrelated change.

Which Ubuntu release is affected by this change?

What kind of change is this? Try to select just one if possible.

• [x ] New feature or improvement
• Backwards-incompatible change, including removed features
• Deprecated feature – will be removed in a later release
• Bug fix
• Known issue
• Something else (please describe)

Workaround

If this is a known issue, how can users work around it for now? Add here if you haven't already described it in the PR.

Which part of Ubuntu does the change affect? Try to select just one if possible.

• [ x] Desktop
• Server
• Containers
• Virtualization
• Databases
• High availability and clustering
• Development tools
• Enterprise
• Cloud
• Security
• WSL
• Real-time Ubuntu
• Hardware support
• A common, underlying change

Major change

Is this a major change? Major changes are highlighted in an overview and are repeated when the next LTS release comes out.

Tickets

Are there any tickets related to the change, e.g. in Launchpad, Jira, GitHub? Paste the links here if you haven't included them in the PR already.

Upstream release notes

Are there any upstream release notes related to the change? Paste the links here if you haven't included them in the PR already.

[msuchane]

Thanks! The limitations were already listed in the known issues section so I replaced them with a reference.