docs(changelog): add #1446 fragment

changelog.d/1446-draw-area-units.md

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+- Show a drawn area of 9,999.5 m² or more as 1.0 ha, and of 999,500 m² or
+  more as 1.00 km². The draw hint picked the unit before rounding, so it read
+  "10000 m²" and "100.0 ha" (Aditya, @Rohilalala, #1446).