docs(binding,docs): clarify .singleton() with .toInstance() behavior in docs and API

- Add an explicit note and warning about the effect (or lack thereof) of calling `.singleton()` after `.toInstance()`:
  - in singleton() API doc-comment in binding.dart,
  - in README.md (after all binding usage patterns),
  - in full_tutorial_en.md and full_tutorial_ru.md.
- Explain that `.singleton()` has no effect on objects registered with `.toInstance()` — they are always single instance.
- Recommend `.singleton()` only for providers (toProvide/toProvideAsync), not direct instances.
- Improves clarity and prevents misuse/confusion for end users and future maintainers.

cherrypick/README.md

@@ -185,6 +185,15 @@ void builder(Scope scope) {
   > Use this pattern only when you want a “master” singleton. If you expect a new instance per params, **do not** use `.singleton()` on parameterized providers.
 
 
+> ℹ️ **Note about `.singleton()` and `.toInstance()`:**
+>
+> Calling `.singleton()` after `.toInstance()` does **not** change the binding’s behavior: the object passed with `toInstance()` is already a single, constant instance that will be always returned for every resolve.
+>
+> It is not necessary to use `.singleton()` with an existing object—this call has no effect.
+>
+> `.singleton()` is only meaningful with providers (such as `toProvide`/`toProvideAsync`), to ensure only one instance is created by the factory.
+
+
 ### Module
 
 A **Module** is a logical collection point for bindings, designed for grouping and initializing related dependencies. Implement the `builder` method to define how dependencies should be bound within the scope.

cherrypick/lib/src/binding.dart

@@ -258,6 +258,13 @@ class Binding<T> {
   /// ```
   ///
   /// Use this pattern only if you want a master singleton. If you expect a new instance per params, **do not** use `.singleton()` on parameterized providers.
+  ///
+  /// ℹ️ **Note about `.singleton()` and `.toInstance()`:**
+  ///
+  /// Calling `.singleton()` after `.toInstance()` does **not** change the binding’s behavior:
+  /// the object passed with `toInstance()` is already a single, constant instance that will always be returned for every resolve.
+  /// There is no need to use `.singleton()` with `toInstance()`. This call has no effect.
+  /// `.singleton()` is only meaningful with providers (`toProvide`, `toProvideAsync`, etc), to ensure only one instance is created by the factory.
   Binding<T> singleton() {
     _resolver?.toSingleton();
     return this;