When a file has multiple licenses, you can mention the license as
"License: Expat and RFC-Reference" and add a
comment section stating the doctrings are licensed under RFC license.
I took a hint for how it's done in gnupg2 package[1]. You could do something
similar here for the description of the RFC, hopefully this should work.
I personally don't care too much about this being mentioned or not, but FTP
master would likely reject the package if copyright is not perfect so here
we are.
[1]: https://salsa.debian.org/debian/gnupg2/-/blob/debian/unstable/debian/copyright#L123