Remove ErrDoc and MsgDoc from the codebase.

This MR boldly removes ErrDoc and MsgDoc from the codebase.

The former was introduced with the only purpose of classifying errors according to their importance, but a similar result can be obtained just by having a simple [SDoc], and placing bullets after each of them.

On top of that I have taken the perhaps controversial decision to also banish MsgDoc, as it was merely a type alias over an SDoc and as such it wasn't offering any extra type safety. Granted, it was perhaps making type signatures slightly more "focused", but at the expense of cognitive burden: if it's really just an SDoc, let's call it with its proper name.

This isn't an erratic patch, but it has to be intended in the context of #18516 , and the removal of ErrDoc was hinted at by @rae here. The removal of MsgDoc is instead "unsolicited" change that I am proposing as I think they might overall lower the cognitive overhead when working with the error types, so I am biting the bullet but if this isn't desirable I am happy to revert that part of the commit and stick with the status quo.

Depends on MR !4728 (closed).