4.0: Add summary of limitations of OCI mode across distributions

[Dave Trudgian]

Some functionality is limited in OCI mode, depending on the underlying distribution, version of crun / runc etc.

We should identify and document the limitations, at least across:

• Ubuntu 20.04 / 22.04
• EL 7 / 8 / 9
• SLES / Leap 12 / 15