Edit docs to clarify RSS process

Add details to the templates/RSS page to clarify that the `rss.xml`
page is only generated if the `generate_rss = true` variable is set
in the site's `config.toml` page.  This information is already present
in other parts of the documentation, but is not present in the RSS page,
which could be confusing.

docs/content/documentation/templates/rss.md

@@ -3,9 +3,12 @@ title = "RSS"
 weight = 50
 +++
 
-Gutenberg will look for a `rss.xml` file in the `templates` directory or 
-use the built-in one. Currently it is only possible to have one RSS feed for the whole
-site, you cannot create a RSS feed per section or taxonomy.
+If the site `config.toml` file sets `generate_rss = true`, then Gutenberg will 
+generate an `rss.xml` page for the site, which will live at `base_url/rss.xml`. To
+generate the `rss.xml` page, Gutenberg will look for a `rss.xml` file in the `templates`
+directory or, if one does not exist, will use the use the built-in rss template.
+Currently it is only possible to have one RSS feed for the whole site; you cannot
+create a RSS feed per section or taxonomy.
 
 **Only pages with a date and that are not draft will be available.**