Added description for allow callback into documentation

[TymeeK]

Please describe your changes

I added a description for the allow callback function in the suggestions documentation.

How did you accomplish your changes

I went into the api/utilities/suggestion.md directory and added my changes

How have you tested your changes

TBD

How can we verify your changes

You can verify the changes by previewing suggestion.md

Remarks

N/A

Checklist

• [x] The changes are not breaking the editor
• [x] Added tests where possible
• [x] Followed the guidelines
• [x] Fixed linting issues

Related issues

https://github.com/ueberdosis/tiptap/issues/4351