docs (content-length) : Add HTTP version comparison (mdn#40534)

* docs (content-length) : Add HTTP version comparison

- Add comparison between HTTP/1.1 and HTTP/2 behaviors
- Addresses issue mdn#36681

* Update files/en-us/web/http/reference/headers/content-length/index.md

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Update files/en-us/web/http/reference/headers/content-length/index.md

* Apply suggestions from code review

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Apply suggestions from code review

* Update files/en-us/web/http/reference/headers/content-length/index.md

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

* MOve to description

---------

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

Author: 4 people

files/en-us/web/http/reference/headers/content-length/index.md

@@ -9,6 +9,12 @@ sidebar: http
 
 The HTTP **`Content-Length`** header indicates the size, in bytes, of the message body sent to the recipient.
 
+`Content-Length` is limited in that the message size must be known up front, before sending the headers, which is a problem when content is dynamically generated or streamed.
+
+- In HTTP/1.0, it is required.
+- In HTTP/1.1, it could be replaced with {{httpheader("Transfer-Encoding", "Transfer-Encoding: chunked")}} for responses sent out in parts as its size is calculated.
+- In HTTP/2, `Content-Length` is redundant, because the content length may be inferred from DATA frames. It may still be included for backwards compatibility.
+
 <table class="properties">
   <tbody>
     <tr>