libstd: improve os::args() doc

src/libstd/os.rs

@@ -1083,6 +1083,10 @@ extern "system" {
 /// Returns the arguments which this program was started with (normally passed
 /// via the command line).
 ///
+/// The first element is traditionally the path to the executable, but it can be
+/// set to arbitrary text, and it may not even exist, so this property should not
+//  be relied upon for security purposes.
+///
 /// The arguments are interpreted as utf-8, with invalid bytes replaced with \uFFFD.
 /// See `String::from_utf8_lossy` for details.
 /// # Example