DOC-2411: Iframes in dialogs were not rendering rounded borders correctly.

[kemister85]

Ticket: DOC-2411

Site: Staging branch

Changes:

• added fix documentation for TINY-10901 to the release notes.

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/

Review:

• [x] Documentation Team Lead has reviewed