Document `$(JavaSdkDirectory)`, `$(AndroidSdkDirectory)`, and `$(AndroidNdkDirectory)`

### Android application type

Classic Xamarin.Android (MonoAndroid12.0, etc.), Android for .NET (net6.0-android, etc.)

### Affected platform version

MS Docs

### Description

We don't list `$(JavaSdkDirectory)`, `$(AndroidSdkDirectory)`, or `$(AndroidNdkDirectory)` here: https://docs.microsoft.com/en-us/xamarin/android/deploy-test/building-apps/build-properties

A customer was looking for these and they are undocumented?!?

Searching MS docs, I only found these mentioned on release notes.

### Steps to Reproduce

Searched via: https://docs.microsoft.com/en-us/search/?terms=javasdkdirectory&scope=Xamarin.Android

### Did you find any workaround?

_No response_

### Relevant log output

_No response_