First, execute sap transaction SE37 and enter the function module name. Then press the display button
Alternatively, choose menu option Goto->Documentation->Function Module Documentation. The only problem I find with this option is that the documentation popup box appears underneath the current SAP window so you don’t actually see it without moving the window
If you selected display you will be taken to the FM details, now simply choose the menu option goto->Documentation.
The FM documentation will now be displayed
You also get the same options to view FM documentation within SAP transaction SE80