Amend RFC 0821: add Resolver public type

text/0821-public-types.md

@@ -37,6 +37,7 @@ Introduce public import locations for type-only imports which have previously ha
     - [`getOwner` and `setOwner`](#getowner-and-setowner)
   - [`RouteInfo`](#routeinfo)
     - [`RouteInfoWithAttributes`](#routeinfowithattributes)
+  - [`Resolver`](#resolver)
 - [How we teach this](#how-we-teach-this)
   - [`Owner`](#owner-1)
   - [`Transition`, `RouteInfo`, and `RouteInfoWithAttributes`](#transition-routeinfo-and-routeinfowithattributes)
@@ -392,6 +393,33 @@ function takesRouteInfoWithAttributes(routeInfoWithAttributes) {
 ```
 
 
+### `Resolver`
+
+The resolver is a contract implemented by libraries outside Ember itself, such as `ember-resolver`, `ember-strict-resolver`, and any number of custom resolvers which exist in apps across the ecosystem. It has never had public documentation, but is fully public API. It is a user-constructible interface with the following definition (using the `Factory` and `FullName` types exported from the new `@ember/owner` module):
+
+```ts
+export type KnownForTypeResult<Name extends string> = {
+  [fullName in `${Name}:${string}`]: boolean | undefined;
+};
+
+export interface Resolver {
+  knownForType?: <Name extends string>(type: Name) => KnownForTypeResult<Name>;
+  lookupDescription?: (fullName: FullName) => string;
+  makeToString?: (factory: Factory<object>, fullName: FullName) => string;
+  normalize?: (fullName: FullName) => string;
+  resolve(name: string): Factory<object> | object | undefined;
+}
+```
+
+The `KnownForTypeResult` utility type associated with it is also available as a named export. Unfortunately, due to currently limitations with TypeScript, you will generally have to *cast* to it, but it provides some type safety to callers, because it will *only* allow types corresponding to the passed string if users pass a string literal.
+
+Both are available as named, type-only, user-constructible interfaces from `@ember/owner`:
+
+```ts
+import { Resolver, KnownForTypeResult } from '@ember/owner';
+```
+
+
 ## How we teach this
 
 These concepts all already exist, but need updates to and in some cases wholly new pages in Ember's API docs.