DOC: more description on the multiline tooltip example
"""
Multi-Line Tooltip
==================
-This example shows how you can use selections and layers to create a multi-line tooltip
-that tracks the x position of the cursor.
+This example shows how you can use selections and layers to create a
+multi-line tooltip that tracks the x position of the cursor.
+
+To find the x-position of the cursor, we employ a little trick: we add some
+transparent points with only an x encoding (no y encoding) and tie a
+*nearest* selection to these, tied to the "x" field.
"""
# category: interactive charts
import altair as alt
