Backtrace generation docs are out of date

[MaxSem]

Instructions from https://bugs.php.net/bugs-generating-backtrace.php don't work anymore.

In modern versions of PHP the following seems to work:

• Printing function name: print (char*)current_execute_data->func.common->function_name.val
• Printing scope name: print (char*)current_execute_data->func.common->scope.name.val

[derickr]

Can you make a pull request? The site is at https://github.com/php/web-bugs