Updated Flutter/Java Basics Documentation

[TylerTrott]

Is your feature request related to a problem? Please describe.
Some devs are going through the SDK Reference portion of the documentation and have had troubles with .get() as the documentation only describes.put().

Describe the solution you'd like
A clear piece of documentation that outlines all methods of working with atKey objects

Describe alternatives you've considered
Just need content :)

Additional context
N/A