Add more precision to the existing description of initial zoom selection

[MAudelGisaia]

For the user, it can be difficult to know the meaning of the value ( ex : a zoom set to 7 is close to or outside the tile ?).

[QuCMGisaia]

For the minimum and maximum zoom of the visibility of the layers, information could also be added about what certain levels of zoom represent, as it is done in mapbox's doc