[DOC] update SHELL_ENV_GENERATORS

[mwichmann]

Doc-only change (including docstrings)

Signed-off-by: Mats Wichmann [email protected]

Contributor Checklist:

• [ ] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [X] I have updated CHANGES.txt (and read the README.rst)
• [X] I have updated the appropriate documentation