[nfc] Add comment describing when there may not be a parent source file.
[nfc] Add comment describing when there may not be a parent source file.

Add a comment explaining that C++ function wont have an attached parent
source file. Refs #35311.

gregomni/swift,tkremenek/swift,atrick/swift,xwu/swift,atrick/swift,tkremenek/swift,hooman/swift,apple/swift,xwu/swift,roambotics/swift,gregomni/swift,glessard/swift,glessard/swift,ahoppen/swift,benlangmuir/swift,atrick/swift,rudkx/swift,ahoppen/swift,parkera/swift,apple/swift,roambotics/swift,hooman/swift,xwu/swift,parkera/swift,parkera/swift,rudkx/swift,gregomni/swift,tkremenek/swift,glessard/swift,glessard/swift,JGiola/swift,xwu/swift,apple/swift,ahoppen/swift,hooman/swift,parkera/swift,roambotics/swift,JGiola/swift,hooman/swift,gregomni/swift,xwu/swift,atrick/swift,roambotics/swift,JGiola/swift,glessard/swift,atrick/swift,benlangmuir/swift,tkremenek/swift,apple/swift,rudkx/swift,apple/swift,gregomni/swift,parkera/swift,xwu/swift,rudkx/swift,hooman/swift,ahoppen/swift,benlangmuir/swift,parkera/swift,glessard/swift,tkremenek/swift,ahoppen/swift,benlangmuir/swift,atrick/swift,rudkx/swift,benlangmuir/swift,parkera/swift,gregomni/swift,JGiola/swift,roambotics/swift,tkremenek/swift,JGiola/swift,parkera/swift,JGiola/swift,rudkx/swift,roambotics/swift,ahoppen/swift,apple/swift,xwu/swift,hooman/swift,hooman/swift,benlangmuir/swift,tkremenek/swift