Clarify float rounding direction for signed zero

Author: orlp

library/core/src/primitive_docs.rs

@@ -1142,10 +1142,9 @@ impl<T: Copy> Copy for (T,) {
 ///     surprising results upon inspecting the bit patterns,
 ///     as the same calculations might produce NaNs with different bit patterns.
 ///
-/// When the number resulting from a primitive operation (addition,
-/// subtraction, multiplication, or division) on this type is not exactly
-/// representable as `f32`, it is rounded according to the roundTiesToEven
-/// direction defined in IEEE 754-2008. That means:
+/// When a primitive operation (addition, subtraction, multiplication, or
+/// division) is performed on this type, the result is rounded according to the
+/// roundTiesToEven direction defined in IEEE 754-2008. That means:
 ///
 /// - The result is the representable value closest to the true value, if there
 ///   is a unique closest representable value.
@@ -1154,6 +1153,9 @@ impl<T: Copy> Copy for (T,) {
 /// - If the true value's magnitude is ≥ `f32::MAX` + 2<sup>(`f32::MAX_EXP` −
 ///   `f32::MANTISSA_DIGITS` − 1)</sup>, the result is ∞ or −∞ (preserving the
 ///   true value's sign).
+/// - If the result of a sum exactly equals zero, the outcome is +0.0 unless
+///   both arguments were negative, then it is -0.0. Subtraction `a - b` is
+///   regarded as a sum `a + (-b)`.
 ///
 /// For more information on floating point numbers, see [Wikipedia][wikipedia].
 ///