Writing Parameter Documentation 
Procedure
Parameter documentation is also written in the Function Builder:
In one session, enter transaction SE37 to display the Function Builder. This is where you write the parameter documentation.
In the second session, enter the transaction BAPI to invoke the BAPI Explorer, and to display the BAPI in the BOR for reference. In the tree view, open the relevant business object and BAPI to display its parameters.
If you cannot find your BAPI in this view, display all BAPIs using the filter "Filter BAPIs according to release status", or start transaction SWO1 and enter the technical name of the business object.
The next steps are carried out in the Function Builder.
If you now compare the parameters in the Function Builder with the parameters in the BAPI Explorer, you may see that there are less parameters displayed in the BAPI Explorer than in the Function Builder.
This is because some BAPIs use the key fields of their business object. Although theses are defined as key fields of the business object in the BOR, they are actually implemented as parameters in the BAPI function module.
In the tree view of the BAPI Explorer, the key fields of a business object are displayed directly above the object’s BAPIs. This is where you will find the "missing" parameters.
You must then copy these short texts in the BOR (transaction SWO1) using the appropriate function Goto -> Documentation. They are then displayed in the BAPI Explorer (Detail tab) and the BOR as the short descriptions of the parameters.

Refer to
Examples of BAPI Documentation.