Fix docs and help messages about OCI image uploads
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Issue body
### Bug Description
The docs and help message about OCI image uploads on Charmcraft 3 are slightly wrong.
See: https://discourse.charmhub.io/t/publish-your-charm-on-charmhub/12281/6?u=lengau
### To Reproduce
https://discourse.charmhub.io/t/publish-your-charm-on-charmhub/12281/6?u=lengau
### Environment
n/a
### charmcraft.yaml
```yaml
n/a
```
### Relevant log output
```shell
n/a
```
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