add comment explaining why the 5k is good

src/providers/twelvedata.zig

@@ -53,6 +53,9 @@ pub const TwelveData = struct {
         const from_str = from.format(&from_buf);
         const to_str = to.format(&to_buf);
 
+        // TwelveData's max outputsize is 5000 data points per request.
+        // For daily candles this covers ~20 years of trading days (~252/year),
+        // well beyond our typical 10-year lookback. No pagination needed.
         const url = try http.buildUrl(allocator, base_url ++ "/time_series", &.{
             .{ "symbol", symbol },
             .{ "interval", "1day" },