Add arguments sharing and isolation section in diagnostic-struct of rdg

[xizheyin]

This PR explains rules for deriving diagnostic shareing arguments in rdg. cc #142724
And fix some nits.

r? @oli-obk

[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

[Kobzol]

Hi, since this PR only modifies the rdg (and not anything else), could you please send it against https://github.com/rust-lang/rustc-dev-guide directly? Thanks!

[xizheyin]

No problem. :) I was advised to submit documentation here the other day. lol
rust-lang/rustc-dev-guide#2465 (comment)

[Kobzol]

Oh, sorry for that 😅 We'll discuss it.

[Kobzol]

I think the confusion mostly was from you not knowing that you can actually land rdg changes in this repo. Normally, you would land them in the same PR that changed/added/removed implementation related to the docs changes, but you sent it as a separate PR, which triggered my original response.

[xizheyin]

I will resubmit this PR in rdg.