Creating a raising edge to wake up the micro controller
What you have encountered is a classic approach vendors take when they run out of pins on a device (or bonding space on silicon.) They merge pin functions.

This is the reason that your WKUP pin is also referred to as PA0-WKUP. You can catch references to both PA0 and WKUP throughout the datasheet. In the end, both physically join in the same pin.



From what I have found now about the use you see on the Discovery board, the pin is used in some sort of power-on/reset voltage monitoring process and WKUP at the same time. In your schematic, there are multiple occurrences of PA0. I'll leave it to you to Ctrl-F all of them.