To maintain the documentation in a report, go to SE38.
Enter the name of the program for which you want to maintain documentation, select the Documentation radio button and press change.
This is the documentation maintain screen. Enter the documentation under relevant sections.
I have maintained the documentation only under PURPOSE section. Now save and activate. Then go back to SE38 initial screen and execute the program.
If the program has documentation maintained, then a blue icon appears next to execute button on the application toolbar. Just press the blue icon to see the documentation of the program.