Improve config documentation for `disallowed_notification_payload_keys` and `notification_payload_transform`

[kaylareopelle]

The new Action Mailer instrumentation (#887) and existing instrumentation for Action View include configuration options named disallowed_notification_payload_keys and notification_payload_transform. They may also be present in other Rails-related instrumentation.

These options don't have a lot of documentation.

We should consider:

• Updating the README to describe these options
• Updating the class comments in the file that contains the options method call to include descriptions

Try to apply the documentation updates to all instrumentation with these configuration options.

[github-actions]

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