complete rewrite of the documentation of AS::PerThreadRegistry

* It focuses on how to use it.

* Removes some ambigueties in the original docs about whether the state is stored in the class.

* Documents it provides class-level accessors via method_missing.

* Documents that if the extended class has an initializer, it must accept no arguments.
simplifies the RDoc of AR::RuntimeRegistry

The previous version was kind of duplicating the documentation of
AS::PerThreadRegistry. Just say how to use it, the thread locals
registry is know part of our vocabulary (though a pointer to
Active Support is added for reference).
removes calls to AR::Runtime.instance

Registries have class-level accessors to write clean code, let's
use them. This makes style uniform also with existing usage in
ScopeRegistry and InstrumentationRegistry.

If performance of the method_missing callback was ever considered to
be a concern, then we should stop using it altogether and probably
remove the callback. But while we have the feature we should use it.
remove unused variable
Merge pull request #10201 from vipulnsward/remove_unused_var_in_guides

remove unused variable