Update docstring about default base/run environments in OCL

[merelcht]

Description

The docstrings weren't updated when we did #3311, which confused a user.

Development notes

Updated the docstrings for base_env and default_run_env arguments, describing the difference between using OmegaConfigLoader directly or through the framework.

Developer Certificate of Origin

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Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

[noklam]

Thank you!