Correctly document is_null CTFE behavior.

The "panic in const if CTFE doesn't know the answer" behavior was discussed to be the desired behavior in rust-lang#74939, and is currently how the function actually behaves.

I intentionally wrote this documentation to allow for the possibility that a panic might not occur even if the pointer is out of bounds, because of rust-lang#133700 and other potential changes in the future.

library/core/src/ptr/const_ptr.rs

@@ -14,12 +14,16 @@ impl<T: ?Sized> *const T {
     ///
     /// ## Behavior during const evaluation
     ///
-    /// When this function is used during const evaluation, it may return `false` for pointers
-    /// that turn out to be null at runtime. Specifically, when a pointer to some memory
-    /// is offset beyond its bounds in such a way that the resulting pointer is null,
-    /// the function will still return `false`. There is no way for CTFE to know
-    /// the absolute position of that memory, so we cannot tell if the pointer is
-    /// null or not.
+    /// When this function is used during const evaluation, this function will
+    /// panic if CTFE cannot determine whether the pointer is null or not. This
+    /// panic might or might not occur when `self` is a pointer that is offset
+    /// beyond the bounds of the memory it initially pointed to. Conversely,
+    /// this panic will never occur when `self` is in-bounds.
+    ///
+    /// This inability to determine nullness occurs because there is no way for
+    /// CTFE to know the absolute position of that memory, so it might or might
+    /// not be possible to determine whether an offset out of bounds happens to
+    /// result in a null pointer.
     ///
     /// # Examples
     ///

library/core/src/ptr/mut_ptr.rs

@@ -14,12 +14,16 @@ impl<T: ?Sized> *mut T {
     ///
     /// ## Behavior during const evaluation
     ///
-    /// When this function is used during const evaluation, it may return `false` for pointers
-    /// that turn out to be null at runtime. Specifically, when a pointer to some memory
-    /// is offset beyond its bounds in such a way that the resulting pointer is null,
-    /// the function will still return `false`. There is no way for CTFE to know
-    /// the absolute position of that memory, so we cannot tell if the pointer is
-    /// null or not.
+    /// When this function is used during const evaluation, this function will
+    /// panic if CTFE cannot determine whether the pointer is null or not. This
+    /// panic might or might not occur when `self` is a pointer that is offset
+    /// beyond the bounds of the memory it initially pointed to. Conversely,
+    /// this panic will never occur when `self` is in-bounds.
+    ///
+    /// This inability to determine nullness occurs because there is no way for
+    /// CTFE to know the absolute position of that memory, so it might or might
+    /// not be possible to determine whether an offset out of bounds happens to
+    /// result in a null pointer.
     ///
     /// # Examples
     ///