I'll leave all changes to the Python interface (or its docs) to @ntamas, but I would phrase it as:

An edge list file contains a series of integer-pairs, where the integers represent vertex indices. Note that indexing is zero-based, therefore the number of vertices in the graph will be the largest integer plus one.

The behaviour is really a consequence of the fact that the integers are vertex indices (vertex IDs) and not vertex named. "Indexing" implies that there is an item for each integer from the smallest (zero) to the largest.

Zero-degree vertices are not really "created". Personally, I find that phrasing misleading. What happens is that if you ever refer to "the 10th vertex" (equivalently, "vertex index 9") that implies that there is also a 1st, 2nd, 3rd, ..., 9th vertex.

I'm okay with the new wording -- technically, @szhorvat is right but this has caused confusion many times in the past (and it still does) so it is probably not pointed out well enough.