com.jme3.anim.tween.action.Action: basic javadoc

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This PR adds a basic documentation for com.jme3.anim.tween.action.Action class.

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I am done with this PR, let me know if there is something else you would like to add or change.

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There were a lot of forum threads requiring and asking for documentation for the animation system. In this PR (and others), I wasted a lot of my time trying to make that happen, so please reward me by looking into it. Thank you!

[scenemax3d]

Hi Pavel,
Can u please change line #152 from "Internal use only" to: "Meant for internal use" ?
I can't change it myself..

Thanks!