Update severity level descriptions in linter-tool.md

[khalilmalla95]

Update the severity level descriptions for clarity:

• ERROR: Critical issues that must be fixed → Critical issue that must be fixed. Will cause the plugin to fail at deployment or runtime.
• WARNING: Issues that should be addressed → Violation of DSF best practices, or something that could be an error depending on context.
• INFO: Informational messages → Technically valid, but something looks unusual or incomplete. Worth a second look — no immediate action required.
• SUCCESS: Validation passed successfully → Validation passed. Shown to confirm what was checked and give confidence in the covered areas.