It sounds like you are confused about what “multi-line” means. A single-line comment ends at the end of the line of source code, and if you want to continue that comment you must put a “///” on the next line. A multi-line comment starts with a “/*” and ends with a “*/” so it can end either on the same line or multiple lines down.
Being “multi-line” says nothing about any how the text within the comment is displayed. To put a line break in an XML comment you must insert a <br/> (“break”) or wrap the line in a <para> (“paragraph”) tag.