You don't need to mention that you will come back to your country at all.
For purpose of travel for my husband (I didn't need it) we wrote that he is going to accompany me until our pr is granted.
It's obvious that we are immigrating to Canada.
No coverletter is needed neither.
All you need is a letter of support, bank letter for proving that you have 40,000 CAD, police certification, Ielts band 5, LMIA exemption form and payment (230 CAD) and personal documents.
And I should mention that the embassy called my office to check my job and salary. Be ready for it.