Add a module comment to util_bug.h

/**

 * \file util_bug.h

 *

 * \brief Macros to manage assertions, fatal and non-fatal.

 *

 * Guidelines: All the different kinds of assertion in this file are for

 * bug-checking only. Don't write code that can assert based on bad inputs.

 *

 * We provide two kinds of assertion here: "fatal" and "nonfatal". Use

 * nonfatal assertions for any bug you can reasonably recover from -- and

 * please, try to recover!  Many severe bugs in Tor have been caused by using

 * a regular assertion when a nonfatal assertion would have been better.

 *

 * If you need to check a condition with a nonfatal assertion, AND recover

 * from that same condition, consider using the BUG() macro inside a

 * conditional.  For example:

 *

 * <code>

 *  // wrong -- use tor_assert_nonfatal() if you just want an assertion.

 *  BUG(ptr == NULL);

 *

 *  // okay, but needlessly verbose

 *  tor_assert_nonfatal(ptr != NULL);

 *  if (ptr == NULL) { ... }

 *

 *  // this is how we do it:

 *  if (BUG(ptr == NULL)) { ... }

 * </code>

 **/

#ifndef TOR_UTIL_BUG_H