docs: add explanation and suggested fix for string enforcement of MemoizedSelector type

projects/ngrx.io/content/guide/migration/v8.md

@@ -56,6 +56,97 @@ const getTodosById = createSelector(
 );
 ```
 
+#### MemoizedSelector enforces the return type to strictly match the second generic type.
+
+For example, prior to 8.0.0 the following would be fine, since the return type `boolean` is widened to `boolean | null`.
+
+ BEFORE:
+
+ ```ts
+export const getLoginPagePending: MemoizedSelector<State, boolean | null> = createSelector(
+  selectLoginPageState,
+  loginState => loginState.pending // boolean
+);
+```
+
+ Now this will produce an error:
+```
+ error TS2322: Type 'MemoizedSelector<State, boolean>' is not assignable to type 'MemoizedSelector<State, boolean | null>'.
+  Types of property 'setResult' are incompatible.
+    Type '(result?: boolean | undefined) => void' is not assignable to type '(result?: boolean | null | undefined) => void'.
+      Types of parameters 'result' and 'result' are incompatible.
+        Type 'boolean | null | undefined' is not assignable to type 'boolean | undefined'.
+          Type 'null' is not assignable to type 'boolean | undefined'.
+```
+
+ Fix would be to specify the type correctly.
+
+ AFTER:
+
+ ```ts
+export const getLoginPagePending: MemoizedSelector<State, boolean> = createSelector(
+  selectLoginPageState,
+  loginState => loginState.pending // boolean
+);
+```
+
+ Another interesting case is when object literals are returned, e.g.
+
+ BEFORE:
+```ts
+interface Reaction {
+  happy: boolean;
+  tweet: string;
+}
+ export const getNews: MemoizedSelector<State, Reaction> = createSelector(
+  newsState,
+  (news) => {
+    if (news.isFake) {
+      return {
+        happy: false,
+        tweet: 'blah blah blah',
+      }
+    }
+    return {
+      happy: true,
+      tweet: 'anyway',
+    }
+  }
+);
+```
+
+ Now the error message would happen (and it is a bit cryptic):
+```
+Type 'MemoizedSelector<State, { happy: false; tweet: string; } | { happy: true; tweet: string; }>' is not assignable to type 'MemoizedSelector<State, Reaction>'.
+  Type 'Reaction' is not assignable to type '{ happy: false; tweet: string; } | { happy: true; tweet: string; }'.
+    Type 'Reaction' is not assignable to type '{ happy: true; tweet: string; }'.
+      Types of property 'happy' are incompatible.
+        Type 'boolean' is not assignable to type 'true'.ts(2322)
+```
+
+ Fix would be to add the return type to the `projector` function
+
+ AFTER: 
+
+ ```ts
+export const getNews: MemoizedSelector<State, Reaction> = createSelector(
+  newsState,
+  (news): Reaction => {
+    if (news.isFake) {
+      return {
+        happy: false,
+        tweet: 'blah blah blah',
+      }
+    }
+    return {
+      happy: true,
+      tweet: 'anyway',
+    }
+  }
+);
+```
+
+
 ### @ngrx/router-store
 
 #### Required usage of `forRoot`
@@ -148,4 +239,4 @@ To stay consistent with the other `@ngrx/*` packages, the following has been ren
 
 The installation of `@ngrx/data` package via `ng add @ngrx/data` will remove `angular-ngrx-data` from the `package.json` and will also perform these renames.
 
-</div>
+</div>