fix missing doc in two APIs

Author: DiuDiu777

library/alloc/src/ffi/c_str.rs

@@ -352,9 +352,13 @@ impl CString {
     /// # Safety
     ///
     /// This should only ever be called with a pointer that was earlier
-    /// obtained by calling [`CString::into_raw`]. Other usage (e.g., trying to take
-    /// ownership of a string that was allocated by foreign code) is likely to lead
-    /// to undefined behavior or allocator corruption.
+    /// obtained by calling [`CString::into_raw`] and this pointer must not be accessed 
+    /// through any other pointer during the lifetime of reconstructed `CString`. 
+    /// Other usage (e.g., trying to take ownership of a string that was allocated by foreign code)
+    /// is likely to lead to undefined behavior or allocator corruption.
+    ///
+    /// This function does not validate ownership of the raw pointer's memory.
+    /// A double-free may occur if the function is called twice on the same raw pointer.
     ///
     /// It should be noted that the length isn't just "recomputed," but that
     /// the recomputed length must match the original length from the

library/alloc/src/str.rs

@@ -604,6 +604,12 @@ impl str {
 /// Converts a boxed slice of bytes to a boxed string slice without checking
 /// that the string contains valid UTF-8.
 ///
+/// # Safety
+///
+/// * The provided bytes must contain a valid UTF-8 sequence.
+///
+/// * The `Box<[u8]>` must have been allocated via the global allocator. 
+///
 /// # Examples
 ///
 /// ```