RFCs are usually protocol specifications. Specifications are usually intended to facilitate interoperability. They document protocols or grammars or some other artifact. Specifications need to be well-written and precise.

The kind of design document that I frequently find superfluous isn't a specification of some protocol, but a prose description of the code one intends to write.

In concrete terms, an RFC might describe TCP header flags and the TCP state machine, but it'd be silent on the Linux kernel's sk_buff structure. A design document would describe sk_buff in detail.