Fixed typos in activesupport [ci skip]

- eval'ed to eval'd in accordance with https://github.com/rails/rails/pull/10889
- tried to improve statement about compiling Procs into methods using define_method
fix ActiveModel::Validations.validators_on doc
Merge pull request #10906 from darai2k/fix-activemodel-validations-doc

fix ActiveModel::Validations.validators_on doc
Merge pull request #10897 from prathamesh-sonpatki/active-support-typos

Fixed typos in activesupport [ci skip]
remove some evals from callback conditionals
reduce evals in depdendent associations
remove evals from the association