Schematize nuanced immutability (conditional resource replacement, downtime, etc.)

Currently, the CloudFormation resource specification and the documentation do not distinguish the difference between an update operation which _fully replaces_ a resource (including data loss) from those updates which cause some other kind of downtime (e.g a server or database reboot).

The documentation maps both of these concepts to 'Update requires: some interruption' and the resource spec maps only the fully immutable replacement operation, as does the schema currently.

We could include a schema property to signal that a property may cause downtime without data loss as a distinguishing concept.

No files, tests, or entry points are named. Compare the CloudFormation resource specification, documentation, and current schema treatment of full replacement versus downtime, then define the distinct semantics and the schema property needed to represent downtime without data loss.

Written by the indexing model from the issue text.