correct the descriptions

Author: DiuDiu777

library/alloc/src/ffi/c_str.rs

@@ -352,7 +352,7 @@ impl CString {
     /// # Safety
     ///
     /// This should only ever be called with a pointer that was earlier
-    /// obtained by calling [`CString::into_raw`] and this pointer must not be accessed
+    /// obtained by calling [`CString::into_raw`] and the memory it points to 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.

library/alloc/src/str.rs

@@ -608,8 +608,6 @@ impl str {
 ///
 /// * The provided bytes must contain a valid UTF-8 sequence.
 ///
-/// * The `Box<[u8]>` must have been allocated via the global allocator.
-///
 /// # Examples
 ///
 /// ```