Add threadsafety note to Dict docstring

[IanButterworth]

It seems helpful to add something like this?

[tkf]

Note that even iterating over Dict is not data-race-free ATM #35835

I'm also personally against an uncoordinated whack-a-mole approach like this. I think a better approach is to clarify what it means to be "thread-safe" (there are possibly multiple kinds of "thread-safety"), document it, and then document methods and/or data structures with a link to the manual section that clarifies what kinds of "thread-safety" that a given usage of a set of methods provides. I'm also assuming that we note that everything is "thread-unsafe" unless documented otherwise.

Until then, maybe notes like this can live in something like a community-driven wiki (e.g., Discourse post) that can be rapidly iterated over.

[IanButterworth]

Closing as the approach above sounds better