Add a disclaimer about x86 f128 math functions

Due to a LLVM bug, `f128` math functions link successfully but LLVM
chooses the wrong symbols (`long double` symbols rather than those for
binary128).

Since this is a notable problem that may surprise a number of users, add
a note about it.

Link: llvm/llvm-project#44744

library/core/src/primitive_docs.rs

@@ -1244,6 +1244,9 @@ mod prim_f64 {}
 /// actually implement it. For x86-64 and AArch64, ISA support is not even specified,
 /// so it will always be a software implementation significantly slower than `f64`.
 ///
+/// _Note: `f128` support is incomplete. Many platforms will not be able to link math functions. On
+/// x86 in particular, these functions do link but their results are always incorrect._
+///
 /// *[See also the `std::f128::consts` module](crate::f128::consts).*
 ///
 /// [wikipedia]: https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format