Update joystick kcm documentation for game controllers

Summary: I couldn't replace every instance of the word joystick with "game controller" because some parts of the documentation only apply to the thumbsticks of the game controller, which are arguably joysticks.

Reviewers: Plasma, Documentation, yurchor

Reviewed By: Documentation, yurchor

Subscribers: yurchor, ngraham, plasma-devel, kde-doc-english

Tags: Plasma, Documentation

Differential Revision: https://phabricator.kde.org/D21925