Wrote JNISinglton docs

I added a few descriptions and links to the empty JNISingleton class doc.

(cherry picked from commit 4eff57d39bb608afdaae8b29f9226481a1520c0b)

doc/classes/JNISingleton.xml

@@ -1,10 +1,13 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="JNISingleton" inherits="Object" version="3.4">
 	<brief_description>
+		Singleton that connects the engine with Android plugins to interface with native Android code.
 	</brief_description>
 	<description>
+		The JNISingleton is implemented only in the Android export. It's used to call methods and connect signals from an Android plugin written in Java or Kotlin. Methods and signals can be called and connected to the JNISingleton as if it is a Node. See [url=https://en.wikipedia.org/wiki/Java_Native_Interface]Java Native Interface - Wikipedia[/url] for more information.
 	</description>
 	<tutorials>
+		<link title="Creating Android plugins">https://docs.godotengine.org/en/3.4/tutorials/plugins/android/android_plugin.html</link>
 	</tutorials>
 	<methods>
 	</methods>