/// The default memory allocator provided by the operating system.
///
/// This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows,
-/// plus related functions.
+/// plus related functions. However, it is not valid to mix use of the backing
+/// system allocator with `System`, as this implementation may include extra
+/// work, such as to serve alignment requests greater than the alignment
+/// provided directly by the backing system allocator.
///
/// This type implements the `GlobalAlloc` trait and Rust programs by default
/// work as if they had this definition: