I do not understand why the Mojang does not explain what its functions are. Since you don't have a web site where some functions are explained and don't always tell our IDE what they do (for example Eclipse tells me what the DeferredRegister does but not what the function ModelRenderer.render does). Why does the Mojang do it?

DeferredRegister is a Type provided by Forge, not by Mojang. Forge code is usually well documented.  
Since the original source code of the game is decompiled, it does not come with comments by default. Comments are usually added via patches, which are a part of Forge.
Mojang might have proper comments and documentation internally, we will likely never know. If they do not choose to share them, that is likely due to legal reasons.

You can contribute to the documentation comments by contributing to the forge project itself. 

Wow thanks, this explain most of my questions.

So, i don‘t want to write a new topic, so i have another question: why Mojang use Java 8 and not the most Version of Java, so Java 13?

Mojang ships the jre if there where to update java every time a new one come out it will cost a lot in bandwidth

and java 8 is LTS so why bother

yes, thanks