Simplify example, improve documentation.

src/ol/source.js

@@ -41,7 +41,9 @@ export {default as Zoomify} from './source/Zoomify.js';
  * @param {import("./tilegrid/TileGrid.js").default} tileGrid Tile grid.
  * @param {function(import("./tilecoord.js").TileCoord): import("./source/Source.js").default} factory Source factory.
  * This function takes a {@link module:ol/tilecoord~TileCoord} as argument and is expected to return a
- * {@link module:ol/source/Source~Source}.
+ * {@link module:ol/source/Source~Source}. **Note**: The returned sources should have a tile grid with
+ * a limited set of resolutions, matching the resolution range of a single zoom level of the pyramid
+ * `tileGrid` that `createFromTileGrid` was called with.
  * @return {function(import("./extent.js").Extent, number): Array<import("./source/Source.js").default>} Sources function.
  * @api
  */