Update documentation for from_raw_parts

Fixes #19269.

Author: steveklabnik

src/libcollections/vec.rs

@@ -195,15 +195,9 @@ impl<T> Vec<T> {
         }
     }
 
-    /// Creates a `Vec<T>` directly from the raw constituents.
+    /// Creates a `Vec<T>` directly from the raw components of another vector.
     ///
-    /// This is highly unsafe:
-    ///
-    /// - if `ptr` is null, then `length` and `capacity` should be 0
-    /// - `ptr` must point to an allocation of size `capacity`
-    /// - there must be `length` valid instances of type `T` at the
-    ///   beginning of that allocation
-    /// - `ptr` must be allocated by the default `Vec` allocator
+    /// This is highly unsafe, due to the number of invariants that aren't checked.
     ///
     /// # Example
     ///