Merge pull request #2220 from Papooch/docs/clarify-scope-hierarchy

docs(provider-scopes): clarify transient scope hierarchy

Author: kamilmysliwiec

content/fundamentals/provider-scopes.md

@@ -48,10 +48,10 @@ Similarly, for [custom providers](/fundamentals/custom-providers), set the `scop
 
 > info **Hint** Import the `Scope` enum from `@nestjs/common`
 
-> warning **Notice** Gateways should not use request-scoped providers because they must act as singletons. Each gateway encapsulates a real socket and cannot be instantiated multiple times.
-
 Singleton scope is used by default, and need not be declared. If you do want to declare a provider as singleton scoped, use the `Scope.DEFAULT` value for the `scope` property.
 
+> warning **Notice** Websocket Gateways should not use request-scoped providers because they must act as singletons. Each gateway encapsulates a real socket and cannot be instantiated multiple times. The limitation also applies to some other providers, like [_Passport strategies_](../security/authentication#request-scoped-strategies) or _Cron controllers_.
+
 #### Controller scope
 
 Controllers can also have scope, which applies to all request method handlers declared in that controller. Like provider scope, the scope of a controller declares its lifetime. For a request-scoped controller, a new instance is created for each inbound request, and garbage-collected when the request has completed processing.
@@ -68,10 +68,12 @@ export class CatsController {}
 
 #### Scope hierarchy
 
-Scope bubbles up the injection chain. A controller that depends on a request-scoped provider will, itself, be request-scoped.
+The `REQUEST` scope bubbles up the injection chain. A controller that depends on a request-scoped provider will, itself, be request-scoped.
 
 Imagine the following dependency graph: `CatsController <- CatsService <- CatsRepository`. If `CatsService` is request-scoped (and the others are default singletons), the `CatsController` will become request-scoped as it is dependent on the injected service. The `CatsRepository`, which is not dependent, would remain singleton-scoped.
 
+Transient-scoped dependencies don't follow that pattern. If a singleton-scoped `DogsService` injects a transient `LoggerService` provider, it will receive a fresh instance of it. However, `DogsService` will stay singleton-scoped, so injecting it anywhere would _not_ resolve to a new instance of `DogsService`. In case it's desired behavior, `DogsService` must be explicitly marked as `TRANSIENT` as well.
+
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 #### Request provider