Occasionally selection screen fields and objects do not show a documentation window at all, and instead simply display the information message:
No documentation available
In this situation, one trick is to use the variant screen for the report. With the report selection screen open, type ctrl+s to enter the variant screen, then press the Technical Name
button to toggle the display of the technical name for all the selection screen fields.
This will only give you the internal parameter name specific to that report. To find more information about the parameter field, you would still need to look at the ABAP code to see how it is defined.