Document the purpose of hierarchy_radio_lvl

[CaroFG]

The README states the following:

hierarchy_lvl1: the h1 of the block
hierarchy_lvl2: the h2 of the block
hierarchy_lvl3: the h3 of the block
hierarchy_lvl4: the h4 of the block
hierarchy_lvl5: the h5 of the block
hierarchy_radio_lvl0: same as hierarchy_lvl0
hierarchy_radio_lvl1: same as hierarchy_lvl1
hierarchy_radio_lvl2: same as hierarchy_lvl2
hierarchy_radio_lvl3: same as hierarchy_lvl3
hierarchy_radio_lvl4: same as hierarchy_lvl4
hierarchy_radio_lvl5: same as hierarchy_lvl5

But after scraping the documentation, we realize that most hierarchy_radio_lvl are null while their corresponding hierarchy_lvl aren't.

It appears that:

• the hierarchy_lvl fields are used to track the overall structure of the document
• the hierarchy_radio_lvl fields are used to identifyi the current section of the page being indexed