chore(docs): Rename ControllerMessenger to Messenger

[Gudahtt]

Explanation

Rename ControllerMessenger to Messenger in the controller docs and examples.

References

Relates to #4538

Changelog

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

[mcmire]

I spotted more instances we may wish to revert in the docs. I guess it is not a huge deal, but it would be good not to mislead people accidentally if they're using the docs as examples (instead of the examples themselves).

[mcmire]

Nit: Do we want to stick with barControllerMessenger here so it matches real code?

[mcmire]

Do we want to continue using SwapsControllerMessenger here so it matches real code? (same for instances of GasFeeMessenger, TokensMessenger, etc. in this document)

[Gudahtt]

Thanks, both good catches. I made the mistake of not self-reviewing after that last update. I've just checked the diff over and I think I've reverted all of the highlighted unnecessary variable renames now.