Document multiple --env-file with example

[dtrudg]

Expand the documentation for the --env-file option to cover multiple --env-files from https://github.com/sylabs/singularity/pull/2855

https://docs.sylabs.io/guides/latest/user-guide/environment_and_metadata.html#env-file-option

The source is in: https://github.com/sylabs/singularity-userdocs/blob/main/environment_and_metadata.rst

The documentation should show an example of how to use multiple environment files. It should also show how a later environment file can override a value set in an earlier one.