[ci skip] Updated where scope to conform to new style
`index_exists?` with `:name` checks specified columns.

[Yves Senn & Matthew Draper]

The column check was embodied in the defaul index name.
If the :name option was used, the specified columns were not verified at all.

Given:

```
assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_yo_momma)
```

That index could have been defined on any field, not necessarily on `:foo_id`.
Merge pull request #16489 from tomkadwill/updating_options_to_new_style

[ci skip] Updated where scope documentation to conform to new style
[ci skip] fix spelling of override
AM, AP, AV, and AMo tests are already order_independent!
Duplicated method in the test helper
Reset ActiveSupport::Dependencies.mechanism to make tests order independent
AS tests are now order_independent!
Fix Railties tests that were order dependent
Railties tests are order_independent! Hopefully.
Merge pull request #16491 from akshay-vishnoi/doc-changes

[ci skip] fix spelling of overridden