[Tagging] units and notations for maxstay

[OSMDoudou]

> I think any discussion of dates and times should start by asking if we could apply ISO8601 to the problem at hand. For example the other thread about start date variants.

By the way, ISO 8601 is already in use at places: https://wiki.openstreetmap.org/w/index.php?search=ISO+8601&title=Special:Search&profile=default&fulltext=1.