~n -> ñ
The m17n control file is self-documenting; see /usr/share/m17n/sa-iast.mim
It's also possible to create a descriptive text in the header in the mim file using the (description ... ) field. Then, if you right-click the Ibus icon and choose preferences/input method/Sanskrit iast/About you will see this description text. Some input method files use this to document the keystrokes. It could be done for sa-iast.mim, but it hardly seems worth it.
Best,
Dominik