Merge pull request #464 from ipfs/gw-trailing-s-dirs

gateway: explain why we normalize to trailing slash in dir URLs

src/http-gateways/path-gateway.md

@@ -520,10 +520,16 @@ This header is more widely used in [SUBDOMAIN_GATEWAY.md](./SUBDOMAIN_GATEWAY.md
 Gateway MUST return a redirect when a valid UnixFS directory was requested
 without the trailing `/`, for example:
 
-- response for `https://ipfs.io/ipns/en.wikipedia-on-ipfs.org/wiki`
- (no trailing slash) will be HTTP 301 redirect with
+- response for UnixFS directory at `https://example.com/ipns/en.wikipedia-on-ipfs.org/wiki`
+ (no trailing slash) MUST be HTTP 301 redirect with
   `Location: /ipns/en.wikipedia-on-ipfs.org/wiki/`
 
+This normalization is required for directory-based relative links
+and security scopes such as Service Worker registrations to work correctly.
+It also ensures the same behavior on path gateways (`https://example.com/ipfs/cid/` with trailing `/`)
+and origin-isolated HTTP contexts `https://cid.ipfs.dweb.link`
+or non-HTTP URLs like `ipfs://cid`, where empty path component is implicit `/`.
+
 ### `X-Ipfs-Path` (response header)
 
 Used for HTTP caching and indicating the IPFS address of the data.