Improve documentation about bounding box

Labelnation sets a clip path for each label, and optionally a bounding box. The code is compact and extremely clever. This commits expands the docstring to explain what the function does and adds a comment to explain why the bounding box is half as wide as one would expect, and subtleties of interaction with inner_margin.

This commit also clarifies the user-facing description of bounding box, that the thin rule is on the label and adjacent to the edge.