Mention that ERR_FILE_EOF is not always an error

Let's look at a naive implementation of error handling:

```gdscript
var fp := FileAccess.open("res://myfile.txt", FileAccess.READ)
var data := fp.get_buffer(65536)
if fp.get_error():
print("PANIK I/O IS BROKEN YOUR HDD IS DYING AAAAAAA")
```

If the file is small enough, `get_buffer` reaches EOF, and `get_error` returns 18 (`ERR_FILE_EOF`).

But unless the user expects exactly 65536 bytes in the buffer, it's not an error, and `ERR_FILE_EOF` can be ignored.

This may be obvious for experienced users, but it's easy to overlook it.

I suggest mentioning somewhere (probably in the `FileAccess` documentation) that reaching EOF when reading dynamically sized data (i.e. when using `get_buffer`, `get_line` and probably other methods) doesn't always indicate something bad and can be safely ignored in some cases.