environment.variables: give an example

nixos/modules/config/shells-environment.nix

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       default = {};
       description = ''
         A set of environment variables used in the global environment.
-        These variables will be set on shell initialisation.
+        These variables will be set on shell initialisation (e.g. in /etc/profile).
         The value of each variable can be either a string or a list of
         strings.  The latter is concatenated, interspersed with colon
         characters.