Create rounded_corner.md

Author: taohsu

components/control/text_area/dsm/guideline/rounded_corner.md

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+## Rounded corner
+
+**`False`** For a square finish.
+
+**`True`** For a finish with rounded corner. To be favored in more emotional, immersive contexts or those tied to specific visual identities. For standard or business-oriented journeys, keep the default corners. This evolution addresses the need for flexibility in adapting the design to certain brand contexts.