Moved explanation of `--mountable` option up

The example code uses `--mountable`. In previous version this was followed by an explanation of the `--full` option, which doesn't make sense. Later on in the text this is better explained. I moved this explanation up without repeating too much.
skip migration check if adapter doesn't support it
Prevent state leak.
Remove redundant code.
use name specified by 'as' for automatic inverse association to avoid reflecting on wrong association
Add missing test cases for #assert_no_difference
Set flash in test session when necessary.

`to_session_value` returns nil when empty.
Fix parsed token value with header `Authorization token=`.
PG arrays should type cast user input

We guarantee that `model.value` does not change after
`model.save && model.reload`. This requires type casting user input for
non-string types.
Merge pull request #15686 from sgrif/sg-pg-int-arrays

PG arrays should type cast user input
Merge pull request #15683 from larrylv/fix-token-with-empty-value

Fix parsed token value with header `Authorization token=`.
Merge pull request #15682 from tgxworld/controller_test_process

Set flash in test session when necessary.
Introduce an Attribute object to handle the type casting dance

There's a lot more that can be moved to these, but this felt like a good
place to introduce the object. Plans are:

- Remove all knowledge of type casting from the columns, beyond a
  reference to the cast_type
- Move type_cast_for_database to these objects
- Potentially make them mutable, introduce a state machine, and have
  dirty checking handled here as well
- Move `attribute`, `decorate_attribute`, and anything else that
  modifies types to mess with this object, not the columns hash
- Introduce a collection object to manage these, reduce allocations, and
  not require serializing the types
Ensure we always define attribute methods
Specifically talking about Rake the library [ci skip]
Add links to RDoc back into API documentation guidelines [ci skip]
These were removed from #15542 unfortunately :(
Builder source code is still available on github [ci skip]
This was also removed from #15542
[ci skip] Clarify ActiveModel::Model docs

ActiveModel attributes initialized with String return String values
Merge pull request #15690 from deeeki/activemodel_model_doc

[ci skip] Clarify ActiveModel::Model docs
Document behavior concerns regarding the full Rails stack [ci skip]
Related to a discussion on #15508