docs(eslint-plugin): no-unnecessary-type-assertion: Add non-null operator example (typescript-eslint#253)

* docs(eslint-plugin): Add non-null operator example

* format

Author: bradzacher

Diff for: packages/eslint-plugin/docs/rules/no-unnecessary-type-assertion.md

@@ -27,6 +27,12 @@ type Foo = 3;
 const foo = 3 as Foo;
 ```
 
+```ts
+function foo(x: number): number {
+  return x!; // unnecessary non-null
+}
+```
+
 Examples of **correct** code for this rule:
 
 ```ts
@@ -37,6 +43,12 @@ const foo = <number>3;
 const foo = 3 as number;
 ```
 
+```ts
+function foo(x: number | undefined): number {
+  return x!;
+}
+```
+
 ### Options
 
 This rule optionally takes an object with a single property `typesToIgnore`, which can be set to a list of type names to ignore.