The "Unpacking Code" section -- https://docs.yoctoproject.org/dev-manual/new-recipe.html#unpacking-code -- should discuss the UNPACKDIR variable, so readers understand what is going on behind the scenes; with the appropriate snippet from bitbake.conf showing how it's used.
Fixed with https://git.yoctoproject.org/yocto-docs/commit/?id=7a95f8900a46d5179f1e1d1b754c0852076df3b5