Elaborate on authoritative sources in Docs Style Guide

doc/development/documentation/styleguide.md

@@ -827,9 +827,40 @@ Using the Markdown extension is necessary for the [`/help`](index.md#gitlab-help
 ### Links to external documentation
 
 When describing interactions with external software, it's often helpful to include links to external
-documentation. When possible, make sure that you are linking to an **authoritative** source.
+documentation. When possible, make sure that you're linking to an [**authoritative** source](#authoritative-sources).
 For example, if you're describing a feature in Microsoft's Active Directory, include a link to official Microsoft documentation.
 
+### Authoritative sources
+
+When citing external information, use sources that are written by the people who created
+the item or product in question. These sources are the most likely to
+be accurate and remain up to date.
+
+Examples of authoritative sources include:
+
+- Specifications, such as a [Request for Comments](https://www.ietf.org/standards/rfcs/) document
+from the Internet Engineering Task Force.
+- Official documentation for a product. For example, if you're setting up an interface with the
+Google OAuth 2 authorization server, include a link to Google's documentation.
+- Official documentation for a project. For example, if you're citing NodeJS functionality,
+refer directly to [NodeJS documentation](https://nodejs.org/en/docs/).
+- Books from an authoritative publisher.
+
+Examples of sources to avoid include:
+
+- Personal blog posts.
+- Wikipedia.
+- Non-trustworthy articles.
+- Discussions on forums such as Stack Overflow.
+- Documentation from a company that describes another company's product.
+
+While many of these sources to avoid can help you learn skills and or features, they can become
+obsolete quickly. Nobody is obliged to maintain any of these sites. Therefore, we should avoid using them as reference literature.
+
+NOTE: **Note:**
+Non-authoritative sources are acceptable only if there is no equivalent authoritative source.
+Even then, focus on non-authoritative sources that are extensively cited or peer-reviewed.
+
 ### Links requiring permissions
 
 Don't link directly to: