docs(typescript-restrictions): clarify non-jsii deps (#3995)

I spent about a day and a half trying to figure this out, so I thought I'd add a little clarification :)

tl;dr: I'm making a CDK Construct package that also includes a custom lambda handler. That handler takes on a non-jsii dep but I couldn't figure out why JSII kept trying to compile that non-jsii dep until I read 
> The API of the *jsii module* can not expose any type from bundled dependencies, since those types would not be available in other languages.
 
about 20 times. 

---

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[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

gh-pages/content/user-guides/lib-author/typescript-restrictions.md

@@ -14,10 +14,14 @@ A *jsii module* can declare dependencies on any other *jsii module* by adding en
 `package.json` file. Since most other platforms do not support multiple different versions of the same library to
 coexist in the same closure, it is recommended to also declare all such dependencies as `peerDependencies`.
 
+### non-jsii dependencies
+
 Occasionally, a dependency on a *non-jsii module* is useful. Since such dependencies do not have generated bindings in
 all the supported languages, they must be bundled with the *jsii module* that depends on them, by adding the library
-into the `bundleDependencies` array in `package.json`. The API of the *jsii module* can not expose any type from bundled
-dependencies, since those types would not be available in other languages.
+into the `bundleDependencies` array in `package.json`. 
+
+The API of the *jsii module* can not expose any type from bundled dependencies, since those types would not be available in other languages.
+TypeScript files that include a non-jsii dependency (e.g. a lambda handler for an AWS CDK Construct) cannot be exported from the `main`/`types` entry point.
 
 !!! info
     For more information on `package.json` file contents, refer to the [npm documentation][package-json].