doc: explain why GitHub merge button is not used

Adds documentation and explicit reasons on why
the GitHub web interface button is not used. This was
explained in the referenced issue by @thealphanerd.

Fixes: nodejs#8893

doc/onboarding.md

@@ -164,8 +164,14 @@ onboarding session.
 
 ## Landing PRs: Details
 
-* Please never use GitHub's green "Merge Pull Request" button.
+* Please never use GitHub's green ["Merge Pull Request"](https://help.github.com/articles/merging-a-pull-request/#merging-a-pull-request-using-the-github-web-interface) button.
   * If you do, please force-push removing the merge.
+  * Reasons for not using the web interface button:
+    * The old merge method will write an ugly commit message.
+    * The old rebase & merge method adds metadata to the commit title.
+    * The latest rebase method changes the author.
+    * The squash & merge method has been known to add metadata to the commit title.
+
 
 Update your `master` branch (or whichever branch you are landing on, almost always `master`)