Added the documentation about length to CString::from_raw

src/libstd/ffi/c_str.rs

@@ -395,6 +395,12 @@ impl CString {
     /// ownership of a string that was allocated by foreign code) is likely to lead
     /// to undefined behavior or allocator corruption.
     ///
+    /// It should be noted that the length isn't just "recomputed," but that
+    /// the recomputed length must match the original length from the
+    /// [`into_raw`] call. This means the [`into_raw`]/`from_raw` methods
+    /// should not be used when passing the string to C functions that can
+    /// modify the string's length.
+    ///
     /// > **Note:** If you need to borrow a string that was allocated by
     /// > foreign code, use [`CStr`]. If you need to take ownership of
     /// > a string that was allocated by foreign code, you will need to