How much do I reveal about the story in my cover letter?

I wrote a short story that is part memoir and part fiction. Through some feedback from one of the Litmags I submitted to for publication, I am pretty sure they do not understand that some of it is fiction. I am submitting the piece as a work of creative nonfiction. Do I detail the fiction parts in my cover letter? Maybe submit it as a fiction piece? Help! I’m brand new to attempting to publish my work.