Get next line documentation incorrect after subject update

[RubenNijhuis]

Currently the docs for get next line are incorrect in a couple ways because the subject has changed since initial writing of the docs.

The prototype is incorrect as it now only requires a file descriptor and the return value has updated:
int get_next_line(int fd, char **line); -> char *get_next_line(int fd);

The return value needs to be updated to say it should return a char pointer with the newline included if there is a newline the line. If there isn't any data to be read anymore it should return a NULL value.