[PATCH dpdk] meson: properly disable docs

[Robin Jarry]

Bruce Richardson, Oct 22, 2024 at 17:25:
> No strong objection to this. Just FYI, the reason it is currently done as
> it is in the build, is that even if you configure a build without the docs
> enabled, you can just build the docs explicitly by doing "ninja -C build
> doc". That is so that one can do a test build of the docs without
> a) having to reconfigure the whole project
> b) not have every build from then on rebuild e.g. the APIs doc if you
> change a header.

Oh, I didn't know it was on purpose. I was surprised to see doc related 
warnings with -Denable_docs=false.

Feel free to disregard this patch if you think it is too drastic.