message: docs: clarify when non-null-assertion-operator is needed in template-syntax
diff: @@ -1935,22 +1935,32 @@ It works perfectly with long property paths such as `a?.b?.c?.d`.
  ### The non-null assertion operator ( <span class="syntax">!</span> )
  -The Angular **non-null assertion operator (`!`)** is a post-fix operator that asserts that the preceeding property path
  -will never be null or undefined.
  +As of Typescript 2.0, you can enforce [strict null checking](http://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html "Strict null checking in TypeScript") with the `--strictNullChecks` flag. TypeScript then ensures that no variable is _unintentionally_ null or undefined.
  -Unlike the [_safe navigation operator_](guide/template-syntax#safe-navigation-operator "Safe naviation operator (?.)")
  -the **non-null assertion operator** does not guard against a null or undefined; rather, it informs the TypeScript type
  -checker that there is something it might be unaware of that ensures that this property path is defined. This prevents
  -TypeScript from reporting that the path is possibly null or undefined when strict null checking is enabled.
  +In this mode, typed variables disallow null and undefined by default. The type checker throws an error if you leave a variable unassigned or try to assign null or undefined to a variable whose type disallows null and undefined.
  -For example, if you use [*ngIf](guide/template-syntax#ngIf) to check if `hero` is defined, you can assert the uses of
  -`hero` are defined in the body of the template.
  +The type checker also throws an error if it can't determine whether a variable will be null or undefined at runtime.
  +You may know that can't happen but the type checker doesn't know.
  +You tell the type checker that it can't happen by applying the post-fix
  +[_non-null assertion operator (!)_]((http://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html#non-null-assertion-operator "Non-null assertion operator").
  +
  +The _Angular_ **non-null assertion operator (`!`)** serves the same purpose in an Angular template.
  +
  +For example, after you use [*ngIf](guide/template-syntax#ngIf) to check that `hero` is defined, you can assert that
  +`hero` properties are also defined.
  <code-example path="template-syntax/src/app/app.component.html" region="non-null-assertion-1" title="src/app/app.component.html" linenums="false">
  </code-example>
  -The Angular **non-null assertion operator (`!`)** is like TypeScript's _non-null assertion operator (!)_
  -introduced in [TypeScript 2.0](http://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html).
  +When the Angular compiler turns your template into TypeScript code,
  +it prevents TypeScript from reporting that `hero.name` might be null or undefined.
  +
  +Unlike the [_safe navigation operator_](guide/template-syntax#safe-navigation-operator "Safe naviation operator (?.)"),
  +the **non-null assertion operator** does not guard against null or undefined.
  +Rather it tells the TypeScript type checker to suspend strict null checks for a specific property expression.
  +
  +You'll need this template operator when you turn on strict null checks. It's optional otherwise.
  +
  ## Summary
  You've completed this survey of template syntax.
  