Don’t tell anyone i suggested you this 
sudo pacman -U https://archlinux.org/packages/core/x86_64/linux-docs/download
You can enjoy the documentation …
Arch linux provides one kernel on stable and a higher version in testing, so when there is a new kernel the docs version will change too. To install the docs from testing
sudo pacman -U https://archlinux.org/packages/testing/x86_64/linux-docs/download
You might have to symlink /usr/lib/modules/5.17.8-arch1-1/build to /usr/lib/modules/5.15.38-2-MANJARO/build in order to work …
Manjaro provides multiple kernels, so not sure how that will be sorted out in the future, if will be available from our repository. It should be an independent/optional package nevertheless, for those that are into kernels.