docs: Clarify the manage_master_user_password_rotation functionality

[magreenbaum]

Currently there is not a way to disable secretsmanager password rotation. There's a workaround which involves initially setting manage_master_user_password_rotation to true, applying, then updating the value to false and reapplying.
I am trying to find the best way to document this limitation and workaround in the repository.

Provider issues:

• [Enhancement]: Disable rotation on RDS managed master password hashicorp/terraform-provider-aws#37779
• [Enhancement]: Secret rotation for manage_master_user_password in RDS with automaticallyafterdays option hashicorp/terraform-provider-aws#33462 (comment)

Related module issues:

• Resolves manage_master_user_password_rotation does not disable rotation #572
• Resolves Database password rotation problem #547
• Resolves Unable to stop automatic key rotation in secret manager when we enable manage master master password using secret manager #538

Description

Motivation and Context

Breaking Changes

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request