minor #14801 Add more details about the ESI detection (javiereguiluz)

This PR was merged into the 4.4 branch.

Discussion
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Add more details about the ESI detection

Fixes #14640.

Commits
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8bd8d51 Add more details about the ESI detection

Author: OskarStark

http_cache/esi.rst

@@ -156,9 +156,9 @@ used ``render()``.
 
 .. note::
 
-    Symfony detects if a gateway cache supports ESI via another Akamai
-    specification that is supported out of the box by the Symfony reverse
-    proxy.
+    Symfony considers that a gateway cache supports ESI if its responses include
+    the ``Surrogate-Capability`` HTTP header and the value of that header
+    contains the ``ESI/1.0`` string anywhere.
 
 The embedded action can now specify its own caching rules entirely independently
 of the master page::