Clear up confusion of log_rotate_size config value

This is a fix for Issue #10776.

Remove the second comment which could be confused with suggesting an invalid value: `// 100 MiB`.

To keep the easily readable example value, it has been moved into the comment header for the config item.

**Edit:** Forgot to mention that if you think other examples need to be added or the wording is a little strange, just let me know - I appreciate the feedback. 
Good idea, should I resubmit the change as a new PR?