docs(xet): document memory-derived download buffer defaults
The three HF_XET_RECONSTRUCTION_DOWNLOAD_BUFFER_* defaults are derived from
usable memory (min of host RAM and container cgroup limit) as of
huggingface/xet-core#943, replacing the fixed 2gb/512mb/8gb constants. High
performance mode is memory-aware too and no longer overrides explicitly set
env vars. Adds the HF_XET_MEMORY_DERIVED_DOWNLOAD_BUFFERS kill switch row.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGuU1K7rpjFfBGrA3SvCxL
