message: docs: add lib build note
  PR Close
diff: @@ -193,3 +193,18 @@ Incremental builds can be run as a background process in your dev environment. T
  <code-example language="bash">
  ng build my-lib --watch
  </code-example>
  +
  +<div class="alert is-important">
  +
  +The CLI `build` command uses a different builder and invokes a different build tool for libraries than it does for applications.
  +
  +* The build system for apps, `@angular-devkit/build-angular`, is based on `webpack`, and is included in all new Angular CLI projects.
  +* The build system for libraries is based on `ng-packagr`. It is only added to your dependencies when you add a library using `ng generate library my-lib`.
  +
  +The two build systems support different things, and even where they support the same things, they do those things differently.
  +This means that the TypeScript source can result in different JavaScript code in a built library than it would in a built application.
  +
  +For this reason, an app that depends on a library should only use TypeScript path mappings that point to the *built library*.
  +TypeScript path mappings should *not* point to the library source `.ts` files.
  +
  +</div>
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