Documment config.log_formatter added on 51aeae91

[ci skip]
Improve Active View Overview guide. [ci skip]
Merge pull request #11401 from dpdawson/master

Improved Active View Overview guide. [ci skip]
move `MissingHelperError` out of the `ClassMethods` module.
add test-case to verify `error.path` when a helper was not found.

This verifies the code added with:
https://github.com/rails/rails/commit/99c9d18601539c7e7e87f26bb047add1f93072af