User feedback to explain missing values component of data explorer column summary statistics

[petetronic]

User feedback suggested it was not immediately obvious what the "missing value" elements (the % and the white/red bar) were in a column's summary statistics display.

Given we had previously rejected the idea of "headers" for the columns of the summary statistics to explain these UI elements, another option is to add tooltips to the elements to explain what they are if the user clicks / focuses or hovers on them.