Describe the API limits

[V-B]

Some API queries are having capacity limits, such as 20 assignments, or 30 user accounts, etc and require using pagination if the total number of returned items is larger than such a number.

Add a comprehensive list of such limits to the Survey Solutions API limits page:
https://docs.mysurvey.solutions/questionnaire-designer/limits/survey-solutions-limits/

Original: surveysolutions/surveysolutions#1302