doc: fix Fast FFI argument count in ffi.md

[watilde]

The text said functions with more than 7 arguments fall back to the generic call path, contradicting the preceding sentence that Fast FFI calls support up to 8 arguments. The implementation only falls back when there are more than 8 arguments (src/ffi/fast.cc), so correct the threshold to 8.

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[ShogunPanda]

Thanks for this.
It's actually a bit more complicated: it's depending on the architecture. Do you mind comparing with docs in https://github.com/nodejs/node/blob/main/doc/contributing/ffi-fast-api-internals.md and making sure it's correctly listed out for each platform. Otherwise I think you can use a generic formula like ”7 or 8 arguments depending on the architecture".

[watilde]

@ShogunPanda Thanks for the careful review, you're right that it's architecture-dependent.

I cross-checked the trampoline generators and ffi-fast-api-internals.md: the hard cap is 8 public arguments (src/ffi/fast.cc), but register pressure lowers the effective count to 7 integer/pointer arguments on AArch64 (src/ffi/platforms/arm64.cc) and 6 on x86-64 (src/ffi/platforms/x64.cc), while floating-point arguments can use up to 8 on both.

Rather than the generic "7 or 8", I reworded the paragraph to spell out the per-architecture limits so the API doc matches the internals doc. Let me know if you'd prefer the shorter generic phrasing instead.