RESUMABLE: consider splitting some text out Section 4.4 (append) to keep focus on interim exchanges

During WGLC review, @MikeBishop raised the following issue on https://lists.w3.org/Archives/Public/ietf-http-wg/2025JulSep/0123.html

> You might consider splitting out part of 4.4 (Append) to a new Upload Completion section, so that 4.4 can focus on handling these interim exchanges.