docs: home.sessionVariables clarification

[rodrigovillarbello]

Description

Closes: https://github.com/nix-community/home-manager/issues/6027

Checklist

• [X] Change is backwards compatible.

• [X] Code formatted with ./format.

• [X] Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• [ ] Test cases updated/added. See example.

• [X] Commit messages are formatted like

  {component}: {description}
  
  {long description}

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • [ ] Added myself as module maintainer. See example.

Maintainer CC

@rycee

[rycee]

Thanks! Merged to master now 🙂