User mapping - Add warning to CSV template

This is a follow-on from #455901 (comment 2025961410).

The current CSV template for user mapping import/export is the following:

Source host,Import type,Source user identifier,Source user name,Source username,GitLab username,GitLab public email
example.com,github,user-a,James,jnutt,jnutt,jnutt@gitlab.com

It could be useful to add a warning to this template not to modify any headers or data other than the two columns to be filled in.

Please do not update the headers or existing values in this sheet,,
Source host,Import type,Source user identifier,Source user name,Source username,GitLab username,GitLab public email
example.com,github,user-a,James,jnutt,jnutt,jnutt@gitlab.com