Reltool copies files I don't want into release

Question

I have the following .config file:

{sys, [
    {lib_dirs, ["/Users/dan/learn_you_some_erlang/erlcount"]},
    {rel, "erlcount", "1.0.0", [
            kernel,
            stdlib,
            {ppool, permanent},
            {erlcount, transient}
        ]},
    {boot_rel, "erlcount"},
    {erts, [
        {mod_cond, derived},
        {app_file, strip}]},
    {profile, embedded}
]}.

and when I run reltool, I get a directory erts-9.1/doc containing a 400-page book about Erlang (among other things). How do I get rid of this?

I tried adding {excl_sys_filters, ["^doc"]} to the erts options, but I get an error

Illegal option: {excl_sys_filters,[\"^doc\"]}