Clarify optional None parameters in docstrings

[IceboxDev]

📝 Description

Docstrings across the cache modules were overly strict about parameter types, omitting that several values—such as expire, tag, or directory could legitimately be None. This led to confusing type-hint warnings. The updated docstrings explicitly document optional None values throughout the core, fanout, Django cache, persistent cache, and recipe helpers, ensuring the documentation matches actual behavior.

✨ Changes

• [x] 🐞 Bug fix (non-breaking change which fixes an issue)
• [ ] 🔨 Refactor (non-breaking change which refactors the code base)
• [ ] 🚀 New feature (non-breaking change which adds functionality)
• [ ] 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] 📚 Documentation update
• [ ] 🔒 Security update