add docs

src/source.ts

@@ -13,6 +13,10 @@ export abstract class Source {
 	/** @internal */
 	_sizePromise: Promise<number> | null = null;
 
+	/**
+	 * Returns promise that resolves with the size of the file.
+	 * If the size is not known, it will be retrieved from the source.
+	 */
 	getSize() {
 		return this._sizePromise ??= this._retrieveSize();
 	}