Add E0019 error explanation

[GuillaumeGomez]

Part of #24407.
cc @michaelsproul

[rust-highfive]

r? @pnkfelix

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[pnkfelix]

The phrase "Bad example" is often used when one is saying that the example itself is bad -- i.e. when the example does not actually serve as a demonstration of the topic at hand.

I would instead write: "Example of erroneous code:" or "Example of non-compiling code:"

[GuillaumeGomez]

I actually had the same comment on the other PRs I opened yesterday. I forgot to update this one, I do it.

[GuillaumeGomez]

Updated!

[pnkfelix]

You should add an additional paragraph about the const fn feature, which is currently feature-gated and thus only available in Nightly (but there are other error explanations like E0152 that reference feature-gated things). The sole purpose of const fn is to be a function call that breaks the rule you wrote here. :)

Here is a demonstration of const fn:

https://play.rust-lang.org/?gist=449fb78958b96eca7528&version=nightly

[pnkfelix]

(I've been discussing the question of whether to include discussion of feature-gated things. For now I think it would be best if @GuillaumeGomez removes the final commit above from this PR, and then we land it. We can then put that final commit up as a separate PR that will only be landed after we resolve the question of how to discuss feature-gated things in error explanations.)