Update Self-Diagnostics troubleshooting readme

[xiang17]

Fixes #2548.

Changes

Please provide a brief description of the changes here.

• Added description that the file is circularly overwritten
• And thus it might not be a proper text file format
• Arranged a few sentences around for better readability

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed