Explain the transitive dependency case

Update import doc to match require doc.

Author: DingoEatingFuzz

docs/rules/no-extraneous-import.md

@@ -4,7 +4,8 @@
 
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-If an `import` declaration's source is extraneous (it's not written in `package.json`), the program works in local, but will not work after dependencies are re-installed. It will cause troubles to your team/contributors.
+If an `import` declaration's source is extraneous (it's not written in `package.json`), the program works in local, but may not work after dependencies are re-installed. It will cause troubles to your team/contributors. If a declaration source is extraneous yet consistently works for you and your team, it may be a transitive dependency (a dependency of a dependency). Transitive dependencies should still be saved as an explicit dependency in `package.json` to avoid the risk of a dependency changing and removing the dependency of a dependency this `import` is relying on.
+
 This rule disallows `import` declarations of extraneous modules.
 
 ## 📖 Rule Details