DB-Level Changefeed: Add db-level changefeed syntax to diagrams.go

When db-level (and schema-level) changefeed is ready to be released, we will need to update `diagrams.go` so our docs show that we support this syntax

Jira issue: CRDB-52107

Start by locating diagrams.go and reviewing how existing changefeed syntax is represented. Once database- and schema-level changefeeds are ready for release, update the diagrams to show both forms; the documentation is done when diagrams.go reflects the released syntax.

Written by the indexing model from the issue text.