Update documentation for tor_spawn_background

#define SPAWN_ERROR_MESSAGE "ERR: Failed to spawn background process - code "

/** Start a program in the background. If <b>filename</b> contains a '/', then

 * it will be treated as an absolute or relative path.  Otherwise the system

 * path will be searched for <b>filename</b>. Returns pid on success, otherwise

 * returns -1.

 * Some parts of this code are based on the POSIX subprocess module from Python

/** Start a program in the background. If <b>filename</b> contains a '/',

 * then it will be treated as an absolute or relative path.  Otherwise the

 * system path will be searched for <b>filename</b>. The strings in

 * <b>argv</b> will be passed as the command line arguments of the child

 * program (following convention, argv[0] should normally be the filename of

 * the executable). The last element of argv must be NULL. If the child

 * program is launched, the PID will be returned and <b>stdout_read</b> and

 * <b>stdout_err</b> will be set to file descriptors from which the stdout

 * and stderr, respectively, output of the child program can be read, and the

 * stdin of the child process shall be set to /dev/null.  Otherwise returns

 * -1.  Some parts of this code are based on the POSIX subprocess module from

 * Python.

 */

static int

tor_spawn_background(const char *const filename, int *stdout_read,