Add clarification on using versionadded / versionchanged directives

docs/contributing/documentation_guidelines.md

@@ -360,7 +360,7 @@ Only use tables when needed, using the [GitHub Flavored Markdown table syntax](h
 
 ### Version added, changed, deprecations
 
-Sphinx offers release-metadata directives to generate this information consistently. Use as appropriate.
+Sphinx offers release-metadata directives to present information about new or updated features in a consistent manner.
 
     ```{versionadded} 2.15
     ```
@@ -382,6 +382,8 @@ Sphinx offers release-metadata directives to generate this information consisten
 
 </details>
 
+These directives will typically be removed two releases after they are added, so should only be used for short-lived information, such as "The `WAGTAILIMAGES_CACHE_DURATION` setting was added". Detailed documentation about the feature should be in the main body of the text, outside of the directive.
+
 ### Progressive disclosure
 
 We can add supplementary information in documentation with the HTML `<details>` element. This relies on HTML syntax, which can be hard to author consistently, so keep this type of formatting to a minimum.