Add documentation comments to Prelude.ard

According to the language reference ([here](https://arend-lang.github.io/documentation/language-reference/prelude)), many declarations in Prelude are treated in a special way, they are not usual language constructs. It would be nice to have documentation comments right in the code to make that more explicit. The language reference is not always at hand, one could fail to find the necessary topics.

Start with Prelude.ard and compare its declarations with the linked language-reference page for Prelude. Add documentation comments where the reference describes special treatment, making those declarations' non-standard status explicit in the source. Done means the relevant special declarations are documented consistently in Prelude.ard.

Written by the indexing model from the issue text.