Add note about decorator loading in Engines guide.

Because decorators aren't referenced by the application, they won't get
autoloaded. And because we recommend the pattern, it would be irresponsible
to not show how to load them properly.

Fixes #10647.
Fix some typo in method names, variables
compatability => compatibility
Merge pull request #10761 from vipulnsward/fix-typo-1

Fix some typo in method names, variables
Merge pull request #10762 from vipulnsward/change_compatibility

compatability => compatibility
add test-case for `Array#to_sentence with blank items.
Merge pull request #10763 from senny/10758_to_sentence_with_blank_elements

test-case for `Array#to_sentence` with `blank?` items.
cleanup whitespace in `active_record/relation.rb`.
`implicit_readonly` is being removed in favor of calling `readonly` explicitly
Merge pull request #10769 from senny/10615_join_should_not_return_readonly_records

using `#joins` does not imply `readonly = true`.
test cleanup, replace `define_method` and `remove_method` with stubs.
Merge pull request #10775 from senny/postgres_adapter_test_cleanup

test cleanup, replace `define_method` and `remove_method` with stubs.