docs: clarify publicly_accessible default for aws_db_instance and aws_rds_cluster_instance

[shashankvarma499]

Description

The documentation for aws_db_instance and aws_rds_cluster_instance states that the publicly_accessible argument defaults to false. The actual default depends on the DB subnet group associated with the instance:

If DBSubnetGroup is specified, PubliclyAccessible defaults to false unless the value of DBSubnetGroup is default, in which case PubliclyAccessible defaults to true.

This updates both resource docs to describe that behavior.

Relations

Closes #46118

[github-actions]

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