docs: Update the merged PR data export information - IC-1628 (#664)

Add information about the new columns and that they are the breakdown of
those numbers that can be seen in the dashboard governance impact donut

docs/infracost_cloud/data_export.md

@@ -30,6 +30,8 @@ The enterprise data export consists of daily CSV files that contain a summary of
 - `infracost_enterprise_branch_summary_YYYYMMDD.csv`: This file shows the current issues from the base branches (e.g. main or master) of all repos across all Infracost orgs, so you can see trends over time. Each file is a complete snapshot of the time it was generated at. This file will show 0 in the issues columns if a repo has no issues. Repos archived and deleted in GitHub/GitLab are not included in this file so the numbers shown will match the Infracost Cloud dashboard. If a repo is renamed, the next day's exported file will reflect the new `repo_name`. 
 - `infracost_enterprise_merged_prs_summary_YYYYMM.csv`: This file shows the cost impact for all pull requests merged in the month. Thus this file has the same data that is used in the cost impact summary in the Infracost Cloud dashboard.
 
+   The issue data provided for this report includes the number of new issues, fixed issues and prevented issues. This data is a breakdown of the numbers that can be seen in the Goverance Impact infographic in the main Infracost Cloud dashboard.
+
    Every day, the file for the current month will be updated with the latest information for the month-to-date, replacing any earlier versions of the file. So for example, on the first day of April 2024, infracost_merged_prs_summary_202404.csv will be created, and on the 2nd of April, the same file will be overwritten with the latest information. On the 1st of May, infracost_merged_prs_summary_202405.csv will be created.
 
    Only repos that have merged pull requests appear in this file. If a repo has no impact data for the current month, it will not be included in this file. If a repo is archived or deleted in GitHub/GitLab, it will still be included in this file as long as it has merged pull requests in the current month. If a repo is renamed, the next day's exported file will reflect the new `repo_name`.