docs(web-components): add notes on TypeScript definition files

Adds implementation notes for TypeScript definition files in
`@carbon/ibmdotcom-web-components` so that new developers can quickly
understand the difference from regular NPM packages.

Refs carbon-design-system#4988.

packages/web-components/IMPLEMENTATION_NOTES.md

@@ -15,7 +15,7 @@
 - [Rendering target](#rendering-target)
   - [Composite components](#composite-components)
   - [Components rendering modal](#components-rendering-modal)
-- [CTA components](#cta-components)<<<<<<< implnotes-video-player
+- [CTA components](#cta-components)
   - [Video CTA](#video-cta)
 - [React integration](#react-integration)
   - [React wrapper generator](#react-wrapper-generator)
@@ -25,6 +25,7 @@
 - [Container components](#container-components)
   - [Triggering action dispatcher](#triggering-action-dispatcher)
 - [Masthead search](#masthead-search)
+- [TypeScript type definition](#typescript-type-definition)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
@@ -252,3 +253,11 @@ The search box feature in masthead consists of two things:
 `<dds-masthead-search-composite>` uses [`ThrottledInputMixin`](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/globals/mixins/throttled-input.ts) to throttle `dds-masthead-search-input` event. `ThrottledInputMixin` calls `_handleThrottledInput()` method, throttled, as the event defined by `eventInput` static method is fired.
 
 `<dds-masthead-search-container>` [intersects the search query with the table of loaded search results](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/components/masthead/masthead-search-container.ts#L64-L70) and [sets `currentSearchResults` property](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/components/masthead/masthead-search-container.ts#L72), which is [rendered as `<dds-masthead-search-item>`](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.13.0/packages/web-components/src/components/masthead/masthead-search-composite.ts#L118-L123).
+
+## TypeScript type definition
+
+All TypeScript code in our source code is transpiled to plain JavaScript code, by a [Babel plugin](https://www.npmjs.com/package/@babel/plugin-transform-typescript). Applications can still refer to our TypeScript types, via `.d.ts` files we ship.
+
+The `.d.ts` files are generated by [regular TypeScript toolchain](https://www.npmjs.com/package/typescript), via [`gulp-typescript`](https://www.npmjs.com/package/gulp-typescript). The build procedure can be found at [here](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/gulp-tasks/build.js#L474-L490).
+
+> 💡 The `.d.ts` for React wrappers are generated by our [custom Babel plugin](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/tools/babel-plugin-create-react-custom-element-type-def.js). An example of such `.d.ts` file can be found [here](https://unpkg.com/@carbon/[email protected]/es/components-react/card/card.d.ts). The build procedure can be found [here](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/blob/v1.15.0/packages/web-components/gulp-tasks/build.js#L388-L397).