Adjust docs slightly so that behavior of width/height fields is documented more precisely.

[john-parton]

…ented more precisely.

Trac ticket number

N/A

Branch description

This is a trivial change which adjust the docs regarding the width and height fields for ImageField.

If you do the following:

1. Save a model with an image field and width/height fields
2. Modify the underlying file outside of django.
3. Load the instance from the DB and re-save it.

Then the width/height fields will not be updated. This might seem obvious to a more experienced dev, but the current documentation isn't exactly clear on the behavior.

See discussion: #18070

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.

[sarahboyce]

Thank you! Pushed small edits 👍