Document That f16 And f128 Hardware Support is Limited (v2)

library/core/src/primitive_docs.rs

@@ -1082,6 +1082,12 @@ impl<T> (T,) {}
 /// bits. Please see [the documentation for [`prim@f32`] or [Wikipedia on
 /// half-precision values][wikipedia] for more information.
 ///
+/// Note that most common platforms will not support `f16` in hardware without enabling extra target
+/// features, with the notable exception of Apple Silicon (also known as M1, M2, etc.) processors.
+/// Hardware support on x86-64 requires the avx512fp16 feature, while RISC-V requires Zhf.
+/// Usually the fallback implementation will be to use `f32` hardware if it exists, and convert
+/// between `f16` and `f32` when performing math.
+///
 /// *[See also the `std::f16::consts` module](crate::f16::consts).*
 ///
 /// [wikipedia]: https://en.wikipedia.org/wiki/Half-precision_floating-point_format
@@ -1181,6 +1187,12 @@ mod prim_f64 {}
 /// as many bits as `f64`. Please see [the documentation for [`prim@f32`] or [Wikipedia on
 /// quad-precision values][wikipedia] for more information.
 ///
+/// Note that no platforms have hardware support for `f128` without enabling target specific features,
+/// as for all instruction set architectures `f128` is considered an optional feature.
+/// Only Power ISA ("PowerPC") and RISCV specify it, and only certain microarchitectures
+/// 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`.
+///
 /// *[See also the `std::f128::consts` module](crate::f128::consts).*
 ///
 /// [wikipedia]: https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format