std: Explain range follows standard half-open range in offset

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

library/core/src/ptr/const_ptr.rs

@@ -386,7 +386,8 @@ impl<T: ?Sized> *const T {
     /// * If the computed offset is non-zero, then `self` must be [derived from][crate::ptr#provenance] a pointer to some
     ///   [allocated object], and the entire memory range between `self` and the result must be in
     ///   bounds of that allocated object. In particular, this range must not "wrap around" the edge
-    ///   of the address space.
+    ///   of the address space. Note that "range" here refers to a half-open range as usual in Rust,
+    ///   i.e., `self..result` for non-negative offsets and `result..self` for negative offsets.
     ///
     /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
     /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.