Clarify AnchorGenerator feature-map mapping

[medisean]

Summary

• clarify that the outer sizes and aspect_ratios tuples map one-to-one to feature maps
• document the shared detection-head requirement for a consistent number of anchors per spatial location
• add concrete single-level and multi-level parameter examples

Fixes #2135

Test plan

• git diff --check
• python3 -m compileall -q torchvision/models/detection/anchor_utils.py
• parsed the module with ast and verified the updated AnchorGenerator docstring

[pytorch-bot]

🔗 Helpful Links

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[medisean]

Signed

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