Fix documentation typos

[Chandraveersingh1717]

Summary

Fixed multiple documentation issues found during code review:

1. Primary fix: imagenet_21k_classes() return value incorrectly documented as character vector (actually returns data.frame)
2. Fixed variable naming typo class_too_idx → class_to_idx
3. Fixed multiple typos and inconsistencies in NEWS.md

Primary Issue

The imagenet_21k_classes() function was documented as:

@return A character vector with 21k entries representing the ImageNet-21k class labels.

But the actual implementation returns:

data.frame(id = ids, label = labels)

[cregouby]

praise thanks @Chandraveersingh1717 for fixing those !