[docs] clarify symbol-sort-key behaviour with and without overlap (m…

…apbox#9379)

src/style-spec/reference/v8.json

@@ -1025,7 +1025,7 @@
     },
     "symbol-sort-key": {
       "type": "number",
-      "doc": "Sorts features in ascending order based on this value. Features with a higher sort key will appear above features with a lower sort key when they overlap. Features with a lower sort key will have priority over other features when doing placement.",
+      "doc": "Sorts features in ascending order based on this value. Features with lower sort keys are drawn and placed first.  When `icon-allow-overlap` or `text-allow-overlap` is `false`, features with a lower sort key will have priority during placement. When `icon-allow-overlap` or `text-allow-overlap` is set to `true`, features with a higher sort key will overlap over features with a lower sort key.",
       "sdk-support": {
         "basic functionality": {
           "js": "0.53.0",