Add note about MaskableEvalCallback

[icheered]

Fix https://github.com/Stable-Baselines-Team/stable-baselines3-contrib/issues/226

Description

Context

• [ ] I have raised an issue to propose this change (required)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (update in the documentation)

Checklist:

• [x] I've read the CONTRIBUTION guide (required)
• [ ] The functionality/performance matches that of the source (required for new training algorithms or training-related features).
• [ ] I have updated the tests accordingly (required for a bug fix or a new feature).
• [ ] I have included an example of using the feature (required for new features).
• [ ] I have included baseline results (required for new training algorithms or training-related features).
• [x] I have updated the documentation accordingly.
• [ ] I have updated the changelog accordingly (required).
• [ ] I have reformatted the code using make format (required)
• [ ] I have checked the codestyle using make check-codestyle and make lint (required)
• [x] I have ensured make pytest and make type both pass. (required)

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