I've found no explanations anywhere about this. And what is "jimu"?
You have to keep in mind that this is not an SDK it is an API so no there is not documentation on internal dijits used in the jimu library. The jimu is a custom esri WAB library of dijits and classes used in WAB. The way to learn about the use of the loadingshelter dijit is to see how esri uses it in there otb widgets. I personally use the loadingshelter in a couple of my widgets and there is not much to it it only have two public method, show and hide and its html template is a simple div with and img and a paragraph element. Let me know if you have some specific question.
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