add explicit warning sign to shell

Author: gfldex

doc/Type/IO.pod6

@@ -329,7 +329,9 @@ output.
 
 Runs a command through the system shell. All shell meta characters are
 interpreted by the shell, including pipes, redirects, environment variable
-substitutions and so on. See L<run|/type/IO#sub_run> if you don't want that.
+substitutions and so on. Shell escapes are a severe security concern and can
+cause confusion with unusual file names. Use L<run|/type/IO#sub_run> if you
+want to be safe.
 
 The return value is of L<type Proc|/type/Proc>.