Add context when changing secret_key_base

[kinduff]

Motivation / Background

The current documentation in https://github.com/rails/rails/blob/main/guides/source/security.md#L118 only mentions that changing secret_key_base will expire currently active sessions, but it does not list other things that will be affected such as encrypted cookies, signed cookies, and Action Text attachments.

This Pull Request aims to add additional context to the documentation to better inform users of the potential impacts of changing the secret_key_base.

Detail

N/A

Additional information

N/A

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.