Is there a strong reason to use a CC license, rather than using dual MIT/Apache 2.0 like the rest of the Rust ecosystem?
Documentation should use the same licenses as code, to allow freely copying it into and out of code without any extra complication. The author of an RFC might want to copy some production Rust code into the RFC to use as an example. And conversely, the author of Rust code may want to copy code or documentation from an RFC into their code.
I would advocate licensing the RFC repository under MIT/Apache 2.0, just like Rust itself.
Also, a couple of minor nits in the proposed COPYRIGHT wording: "respective writers" should be "respective writers and contributors" (since some RFCs incorporate contributions by others), and "owned by" should say "copyrighted by" or similar. And "All RFCs in this repository" wouldn't apply to anything in the repository other than RFCs (such as the top-level documentation).