docs: recommended GGML EP config (sync from PR #2285)

[KakaruHayate]

Synchronous copy of the recommended GGML configuration comment added to PR openutau#2285.

Reference: the 7-channel 60 s benchmark in game.cpp repo (docs/benchmark-7channel.md):

• GGML F32 vs Q8 identical quality (presence F1~0.998, RPA 0.97+ frame-level)
• CPU DBCache cuts wall -43~44%; GPU prefers F32, Vulkan smallest VRAM
• Q8 may flip a boundary note on Vulkan (1-note flip) — full package for bit-consistency

Code-only: adds a doc comment to GameGgmlBackend.cs, no behavior change.

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