docs(changelog): add the range ring visibility fragment for #1518

changelog.d/1518-range-ring-visibility.md

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+- Draw the Global Context 250 km range ring as a 4 px line with a dark halo,
+  so it stays visible over desert, snow, ocean and forest and under the FLIR,
+  NVG and CRT styles, where the old 1 px outline disappeared (manjunath22466,
+  #1518).