Allow Content-Length to be omitted when pushing on patch requests

Technically due to the way the specification is written today the moby/docker
cli push is nonconformant. Given that is one of the most  common ways users
are pushing today, the specification should recognize that it is allowed.

Further, requiring `Content-Length` to be set means that no client that
is conforming can use http chunked encoding, which limits the use of
CDNs like CloudFlare and other reverse proxies. See further discussion
at: containerd/containerd#7459

Signed-off-by: Ian Zink <ian@replicated.com>

spec.md

@@ -328,7 +328,7 @@ OCI-Chunk-Min-Length: <size>
 Please reference the above section for restrictions on the `<location>`.
 
 ---
-To upload a chunk, issue a `PATCH` request to a URL path in the following format, and with the following headers and body:
+To upload a chunk, issue a `PATCH` request to a URL path in the following format, and with the following headers and body. `Content-Length` can be omitted if HTTP chunked transfer encoding is used:
 
 URL path: `<location>` <sup>[end-5](#endpoints)</sup>
 ```