chore: improve warning message for returning a constant value

[TomAFrench]

Description

Problem*

Resolves

Summary*

This message is a bit strong for a warning (constant return values are technically allowed, there's just generally not much reason for them). It also doesn't give much information on why they may not be a good idea.

I've updated the warning to something which I think is more useful. Happy to take feedback on this however.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

Tagging Guillaume as he added this warning originally.