Updating them to libclang-13-dev was a good change, but we can go even
further:
* The suggestion didn't apply to MacOS.
* The suggestion would need to be updated with every LLVM release,
which is a maintenance burden.
* The suggestion is wrong when compiling with `-tags=llvm12` for
example to choose a different LLVM version.
Therefore, link to the build documentation instead.
320f21524e