Update documentation of image_size_in_gb

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Description

After discussing on slack, it appears image_size_in_gb's behaviour has changed with the release of unified snapshots..

Would it be possible to update de docs to clarify how this parameter works.
Right now here are the questions I have a hard time answering:

• Is it a fixed size the image will take, or just a maximum ?
• How does this parameter influences the type of OS disk that is used (local vs block storage) ?

Use Case(s)

Improve shutdown time and overall packer build times

Potential References

https://scaleway-community.slack.com/archives/C03B4GW7DFG/p1685450458343039