Improve allow_deprecated_singular_associations_name config doc [ci-skip]
Improve allow_deprecated_singular_associations_name config doc [ci-skip]

Follow-up to #45344.

This tweaks the description, fleshes out the code example, and fixes the
default value listed for 7.1.

MSP-Greg/rails,MSP-Greg/rails,yahonda/rails,georgeclaghorn/rails,rails/rails,yawboakye/rails,esparta/rails,rails/rails,Erol/rails,BlakeWilliams/rails,Erol/rails,MSP-Greg/rails,georgeclaghorn/rails,BlakeWilliams/rails,esparta/rails,Erol/rails,yahonda/rails,BlakeWilliams/rails,yawboakye/rails,yawboakye/rails,georgeclaghorn/rails,Erol/rails,rails/rails,georgeclaghorn/rails,yahonda/rails,esparta/rails,yahonda/rails,yawboakye/rails,MSP-Greg/rails,esparta/rails,BlakeWilliams/rails,rails/rails