The tarballs with Octave’s sources already contains a built documentation. When building from a tarball, the documentation doesn’t need to be rebuilt. And usually, the build system skips rebuilding the documentation when it is already there.
However, sometimes it is necessary to make minor modifications to the sources in the tarball to accommodate distribution-specific requirements. I’d guess in about 100% of these cases - at least in principle - it wouldn’t be necessary to rebuild the documentation.
However - dependent on which files are modified - those changes might trigger the build system to rebuild the documentation. That requires a set of additional build time dependencies (some of which might be missing or buggy on the build system).
It is possible to skip building the documentation by configuring with
--disable-docs. But in that case, the documentation isn’t installed either.
Would it be possible to skip building the documentation but still install it when it is already there?