Add note about npm package names requiring scope higher up in docs

doc/user/packages/npm_registry/index.md

@@ -102,6 +102,7 @@ To authenticate to the Package Registry, you must use one of the following:
 - It's not recommended, but you can use [OAuth tokens](../../../api/oauth2.md#resource-owner-password-credentials-flow).
   Standard OAuth tokens cannot authenticate to the GitLab NPM Registry. You must use a personal access token with OAuth headers.
 - A [CI job token](#authenticate-with-a-ci-job-token).
+- Your NPM package name must be in the format of [@scope:package-name](#package-naming-convention). It must match exactly, including the case.
 
 ### Authenticate with a personal access token or deploy token
 
@@ -240,6 +241,7 @@ Prerequisites:
 
 - [Authenticate](#authenticate-to-the-package-registry) to the Package Registry.
 - Set a [project-level NPM endpoint](#use-the-gitlab-endpoint-for-npm-packages).
+- Your NPM package name must be in the format of [@scope:package-name](#package-naming-convention). It must match exactly, including the case.
 
 To upload an NPM package to your project, run this command:
 
@@ -455,3 +457,11 @@ If you get this error, ensure that:
 - The scoped packages URL includes a trailing slash:
   - Correct: `//gitlab.example.com/api/v4/packages/npm/`
   - Incorrect: `//gitlab.example.com/api/v4/packages/npm`
+
+### `npm publish` returns `npm ERR! 400 Bad Request`
+
+If you get this error, your package name may not meet the
+[@scope:package-name package naming convention](#package-naming-convention).
+
+Ensure the name meets the convention exactly, including the case.
+Then try to publish again.