Clarify docs for static files in collection (#7812)

Merge pull request 7812

docs/_docs/collections.md

@@ -45,9 +45,12 @@ Create a corresponding folder (e.g. `<source>/_staff_members`) and add
 documents. Front matter is processed if the front matter exists, and everything
 after the front matter is pushed into the document's `content` attribute. If no front
 matter is provided, Jekyll will consider it to be a [static file](/docs/static-files/)
-and copy it to the destination (e.g. `_site`) without processing. If front matter
-is provided, Jekyll will process the file in your collection but will not write to disk
-unless `output: true` is set in the collection's metadata.
+and the contents will not undergo further processing. If front matter is provided,
+Jekyll will process the file contents into the expected output.
+
+Regardless of whether front matter exists or not, Jekyll will write to the destination
+directory (e.g. `_site`) only if `output: true` has been set in the collection's
+metadata.
 
 For example here's how you would add a staff member to the collection set above.
 The filename is `./_staff_members/jane.md` with the following content:
@@ -60,6 +63,12 @@ position: Developer
 Jane has worked on Jekyll for the past *five years*.
 ```
 
+<em>
+  Do note that in spite of being considered as a collection internally, the above
+  doesn't apply to [posts](/docs/posts/). Posts with a valid filename format will be
+  marked for processing even if they do not contain front matter.
+</em> 
+
 <div class="note info">
   <h5>Be sure to name your directories correctly</h5>
   <p>

docs/_docs/upgrading/3-to-4.md

@@ -53,6 +53,13 @@ your pages and documents.
 The downside to this is that some of the community-authored plugins may not work
 as they previously used to.
 
+## Static files in unrendered collections
+
+Collections other than `posts` can contain static assets along with Markdown files.
+But if the collection has not been configured with metadata `output: true`, then
+neither its *documents* nor its *static assets* will be output to the destination
+directory.
+
 ## For plugin authors
 
 * If your plugin depends on the following code: `site.liquid_renderer.file(path).parse(content)`,