Expound on Importing Issues from CSV doc

Problem to solve

• What product or feature(s) affected? Importing Issues from CSV
• What docs or doc section affected? https://docs.gitlab.com/ee/user/project/issues/csv_import.html
• Is there a problem with a specific document, or a feature/process that's not addressed sufficiently in docs?
  1. Please make a clearer point to users that the issue title and description must be the first two columns (in that order) in the csv file otherwise that data will not import.
  2. Please also make a caution to the reader that there is no current way to mass delete issues so we recommend importing a test file of 1 or 2 issues prior to importing a large file.