docs: be more precise on what `spack add ...` does (#23204)

This is as much a question as it is a minor fine-tuning of the docs.  I've been known to add things to an environment by editing the `spack.yaml` file directly.  When I read the previous version of this sentence, I was afraid that `spack add` was actually doing *two* things, modifying the `spack.yaml` and updating something else that defined the roots of the Environment.  A bit of experimentation suggests that editing the `spack.yaml` file is sufficient to change the roots.

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

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