doc: fix Fast FFI argument count in ffi.md

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 real limit is architecture- and type-dependent: 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 and 6 on
x86-64, while floating-point arguments can use up to 8 on both. Describe
this instead of the previous, inaccurate single threshold.

Signed-off-by: Daijiro Wachi <daijiro.wachi@gmail.com>
PR-URL: #63960
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>

Author: watilde

doc/api/ffi.md

@@ -125,8 +125,13 @@ raw pointer `bigint` values. For pointer-like parameters, `null`, `undefined`,
 strings, `Buffer`, typed array, `DataView`, and `ArrayBuffer` values are
 converted on the JavaScript side before calling the optimized native wrapper.
 
-Optimized Fast FFI calls support at most 8 function arguments. Functions with
-more than 7 arguments use the generic FFI call path instead.
+Optimized Fast FFI calls support at most 8 function arguments, but the exact
+limit depends on the architecture and on the argument types, because each
+argument must fit in the registers used by the platform trampoline. Integer
+and pointer arguments are limited to 7 on AArch64 and to 6 on x86-64, while
+floating-point arguments can use up to 8 on both. Functions that exceed these
+limits, including any function with more than 8 arguments, use the generic FFI
+call path instead.
 
 ## Signature objects