Revise wording in Rc documentation.

[lambdacat]

The term "thread-local" has a widely accepted meaning which is not
the meaning it's used for here.

[rust-highfive]

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[aturon]

Thanks, good catch!

For the sake of non-concurrency experts, I wonder if we can take your wording, but then also add a clarification -- "usable only within a single thread", or something like that.

[GuillaumeGomez]

I'm not sure this new wording is better. I think a longer one would fit the most.

[apasel422]

What's the status of this?

[GuillaumeGomez]

@apasel422: No news from @arbitrary-cat, so no idea...

[lambdacat]

How's that?

[steveklabnik]

Looks great to me, thanks for coming back to this!

@bors: r+ rollup

[bors]

📌 Commit 46e7c9e has been approved by steveklabnik