[DBInstance] Remove documentation for read only CertificateDetails property

[angusy29]

Description of changes:

• [DBInstance] Discovered the public documentation incorrectly displayed CertificateDetails as a property users can mutate, however CertificateDetails is just a readonly property

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[zrfr]

The contents of the docs folders are automatically generated from the schema whenever "cfn generate" is run (this is done as part of the build).

It looks like it automatically omits read-only properties but not the parents of nested properties, like what we have here. Maybe this is more of a CFN issue? (https://github.com/aws-cloudformation/cloudformation-cli/blob/d030eea7c0e68b7ac6ad696f33e623afc85cb5a2/src/rpdk/core/project.py#L981-L985, https://github.com/aws-cloudformation/cloudformation-cli/blob/master/src/rpdk/core/templates/docs-readme.md)