minor #10907 Document the multi-host DSN for Memcache caches (javiereguiluz)

This PR was squashed before being merged into the 4.2 branch (closes #10907).

Discussion
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Document the multi-host DSN for Memcache caches

Fixes #10402.

Commits
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1ae951b Document the multi-host DSN for Memcache caches

Author: javiereguiluz

components/cache/adapters/memcached_adapter.rst

@@ -64,6 +64,16 @@ helper method allows creating and configuring a `Memcached`_ class instance usin
         // etc...
     ]);
 
+    // a single DSN can define multiple servers using the following syntax:
+    // host[hostname-or-IP:port] (where port is optional). Sockets must include a trailing ':'
+    $client = MemcachedAdapter::createConnection(
+        'memcached:?host[localhost]&host[localhost:12345]&host[/some/memcached.sock:]=3'
+    );
+
+.. versionadded:: 4.2
+
+    The option to define multiple servers in a single DSN was introduced in Symfony 4.2.
+
 The `Data Source Name (DSN)`_ for this adapter must use the following format:
 
 .. code-block:: text