docs: update docs for Single.fromFuture

lib/src/single/as_single.dart

@@ -4,9 +4,10 @@ import 'single.dart';
 
 /// Provides [asSingle] extension for [Future].
 extension AsSingleFutureExtension<T> on Future<T> {
-  /// Converts this [Future] into a [Single].
+  /// Converts this [Future] into a [Single]. It is a shorthand for [Single.fromFuture].
   ///
-  /// See [Single.fromFuture].
+  /// See also [Single.fromCallable] and [Single.defer]. These methods are useful
+  /// or creating [Single]s that don't instantiate [Future]s until they are listened to.
   Single<T> asSingle() => Single.fromFuture(this);
 }
 

lib/src/single/single.dart

@@ -118,6 +118,9 @@ class Single<T> extends StreamView<T> {
   ///
   /// NOTE: x is error event
   /// ```
+  ///
+  /// See also [Single.fromCallable] and [Single.defer]. These methods are useful
+  /// for creating [Single]s that don't instantiate [Future]s until they are listened to.
   factory Single.fromFuture(Future<T> future) =>
       Single.safe(Stream.fromFuture(future));