Replaced the `isDocs` specialArg added in #1807 with an internal module
option.
We should only use `specialArgs` when absolutely necessary, a module or
`_module.args` is preferred, but those aren't available until `config`
is evaluated so they can't be used for `imports`.
The main problem with `specialArgs` is it makes our modules less
portable.
Luckily that shouldn't be an issue for these context flags.
