Describe commands useful for updating source trees

Closes #7184, #8369

cc: @staticfloat @tkelman
Should probably elaborate a bit more here. When we add a patch to a dependency, just doing `make -C deps clean-llvm` won't actually apply the patch, we will just rebuild the existing unpatched sources again. `make -C deps distclean-llvm` will delete the downloaded sources and tarball, so the next build will re-download them, ensuring any new patches get applied.