docs(mem_reader): sync MemReader backends, fine_transfer, and config fields

[CaralHsi]

Summary

Align MemReader documentation with current MemOS source code. Updates include:

• Added explicit backend names (simple_struct, multimodal_struct) to class references
• Fixed fine_transfer_simple_mem parameter description (expects nested windows, not flat list)
• Updated read_multi_modal/ parser list to match actual parsers (ImageParser, FileContentParser, ToolParser)
• Replaced outdated chat_window_max_tokens top-level config with chat_chunker factory config
• Added undocumented config fields: general_llm, image_parser_llm

Verification

• All referenced classes exist in code (Check 1)
• Import paths verified (Check 2)
• Method signatures match code (Check 3)
• Config fields match code (Check 4)
• N/A - no API endpoint changes (Check 5)

Cross-Language

• Chinese (content/cn/) counterpart needs separate update

Code Reference

Based on MemOS main branch HEAD

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