Better deal with year integer?

[kanitw]

Hi Jake,

https://stackoverflow.com/questions/55082460/strange-year-values-on-x-axis/55082695#55082695 seems like a common problem that users doesn't know that integer could be a year.

We should at least document about it in https://altair-viz.github.io/user_guide/times_and_dates.html for now.

I'm also filing an issue in Vega to see if we can fix it there. vega/vega#1681

[joelostblom]

Just a note about re-opening. This is not closed by #3218 and should remain open until we know if vega/vega#1681 will be implemented in Vega or if should create a workaround in Altair for compatibility with Vegafusion as per vega/vegafusion#402