[luatex] Documentation improvement about open_read_file callbacks and readers

[Jérôme LAURENS]

The documentation about readers (p 172 for v1.15) is

```
Your function should return either a string or nil. The value nil signals that the end of file has
occurred, and will make TEX call the optional close function next.

```
This is partly exact. If the reader returns nil since the very first call, then it is called once again and only then the close function is eventually called. The reader is always called at least twice. It is worth mentioning because it may be counterintuitive.