DOC: update docs to show construction of periodindex when needing out-of bounds spans
cc @CarstVaartjes

this what you had in mind?
 great improvement in terms of clarity! 

While in general I do understand the period_range, what made it click for me was the applymap with Period (as it made me understand how to create a Period column that you can use from your existing data).
I'll try to make nice docstrings for the to_periods that also explains that concisely
