added documentation for PolylineEncoder

Author: Petr Sramek

src/PolylineAlgorithm/PolylineEncoder.cs

@@ -45,11 +45,16 @@ public Polyline Encode(IEnumerable<Coordinate> coordinates) {
 
         return writer.ToPolyline();
 
+        /// <summary>
+        /// Gets the count of coordinates in the enumerable.
+        /// </summary>
+        /// <param name="coordinates">The enumerable of coordinates.</param>
+        /// <returns>The count of coordinates.</returns>
         [ExcludeFromCodeCoverage]
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         static int GetCount(ref readonly IEnumerable<Coordinate> coordinates) => coordinates switch {
             ICollection<Coordinate> collection => collection.Count,
             _ => coordinates.Count(),
         };
     }
-}
+}