[Docs]: fix and improve equinix_fabric_connection documentation

[ocobles]

To create a redundant fabric v4 connection you need to create a connection with redundancy.priority = PRIMARY and then specify the redudancy.group retrieved from the primary connection in the secondary connection. The schema is fine but the documentation says it's a ready-only field: https://github.com/equinix/terraform-provider-equinix/blob/2855e91fa89f1545de741eacff57ecaed171f849/docs/resources/equinix_fabric_connection.md?plain=1#L481-L483

There may be other wrong fields, and we should provide some examples in the documentation or add links to those available in the repo

[ocobles]

According to web documentation, the Note about IBM additional info configuration https://registry.terraform.io/providers/equinix/equinix/latest/docs/resources/equinix_fabric_connection#nested-schema-for-a_sideadditional_info should be in the top level additional_info field instead

[ocobles]

a_side.access_point.interface.id must be optional instead of read_only

https://registry.terraform.io/providers/equinix/equinix/latest/docs/resources/equinix_fabric_connection#nestedblock--a_side--access_point--interface