docs(rds): add example + note for blue_green_update block on aws_db_instance (#44488)

[GabrielOvie]

What

Add a small HCL example for blue_green_update on aws_db_instance and a short note that it is a block (not an argument).

Why

Users often hit an “Unsupported argument” error when using blue_green_update = { ... }. This shows the correct block syntax and calls it out.

Linked issue

Fixes #44488

Scope

Docs-only change.

[github-actions]

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