As of #18260, `spack load` and `spack env activate` now use
`prefix_inspections` from the modules configuration to decide
how to modify environment variables.
This updates the modules configuration documentation to describe
how to update environment variables with the `prefix_inspections`
section. This also updates the `spack load` and environments
documentation to refer to the new `prefix_inspections` documentation.
c9ad2affcc