add src/librustdoc and src/rustdoc-json-types to RUSTC_IF_UNCHANGED_ALLOWED_PATHS

[lolbinarycat]

this constant (in src/bootstrap/src/core/config/config.rs) list what paths can be changed without affecting the produced rustc binary.

since rustdoc isn't embedded in the compiler, shouldn't these be on the list?

[Noratrieb]

I think this would be sound, yes. Given that working on rustdoc is a major use case for download-rustc, it makes a lot of sense to add them to the list (a list that should generally be kept as minimal as possible).

[onur-ozkan]

We decided to start with smallest possible list, there are various paths that can be included in the list.

Also, I would share these open-ended topics/questions on Zulip so that people who might have an opinion on it have a better chance of seeing it.

[lolbinarycat]

I don't open these for open ended discussion. I open them mostly as a reminder to myself (since I usually run into these things when I'm doing something else), and also partially to allow feedback from team members, such as if there are any objections to the change.

[onur-ozkan]

When it ends with a question and expecting some feedback from others (there are some people that have interest in the behavior of the CI rustc if-unchanged logic), it seems quite open-ended to me. Anyway...