docs(web): review keyboard metadata description 🎼

We might want to review and improve the descriptions of the keyboard metadata in [getKeyboard.md](https://github.com/keymanapp/keyman/blob/epic/web-core/web/docs/engine/reference/core/getKeyboard.md) and [keyboardDetails.ts](https://github.com/keymanapp/keyman/blob/epic/web-core/web/src/app/browser/src/keyboardDetails.ts).

Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/15856:

> The struct returned itself seems to have a lot of loose edges and the capitalization scheme is very outdated!

LanguageCode::
```diff
- * The three-letter code used to internally represent the language.
+ * The ISO639 code used to internally represent the language.
```

> Not sure if this is true though? It could be two or three letters, right?

InternalName:

> Might be good to describe what the InternalName looks like - id of the keyboard, does it include `Keyboard_`?

RegionCode:
```diff
- * The three-letter code representing the region.
+ * The ISO3166 two-letter code representing the region.
```

CountryCode:

> What's the difference between `CountryCode` and `RegionCode`?