It is up to you whether you want to mention the term 'internship', but it is important to mention the following - all positions held while employed at the company and must include the following details: job title, duties and responsibilities, job status (if current job), dates worked for the company, number of work hours per week and annual salary plus benefits.
If you have the reference letter in the format as mentioned here http://www.cic.gc.ca/english/resources/tools/perm/express/intake-complete.asp you are good with just the reference letter. But if there is anything missing like for example pay details etc, then you can upload last 3 or 6 months' payslip or any other relevant supporting document(s) of each job for which you are claiming points.