It is a simple process to retrieve the SAP function module documentation available within your SAP system and can be done via SAP transactions SE80 or SE37.
Step 1 – Execute transaction SE37
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
Step 2 – Display Function Module documentation
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
Step 3 – Alternate way to display documentation for an FM using transaction SE80
You also get the same options to view FM documentation within SAP transaction SE80
