doc: state that Float is IEEE compliant (#3157)

Github discussion:
https://github.com/leanprover/lean4/pull/3147#discussion_r1446735973

src/Init/Data/Float.lean

@@ -26,7 +26,8 @@ opaque floatSpec : FloatSpec := {
   decLe := fun _ _ => inferInstanceAs (Decidable True)
 }
 
-/-- Native floating point type, corresponding to 64-bit `double` in C. -/
+/-- Native floating point type, corresponding to the IEEE 754 *binary64* format
+(`double` in C or `f64` in Rust). -/
 structure Float where
   val : floatSpec.float