minor #17475 Mark some Symfony CLI feature as experimental (javiereguiluz)

This PR was merged into the 4.4 branch.

Discussion
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Mark some Symfony CLI feature as experimental

As asked by `@fabpot` in symfony-cli/symfony-cli#236 (comment)

Commits
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1d78eb9 Mark some Symfony CLI feature as experimental

Author: javiereguiluz

setup/symfony_server.rst

@@ -280,6 +280,11 @@ server provides a ``run`` command to wrap them as follows:
 Configuring Workers
 -------------------
 
+.. caution::
+
+    This feature is experimental and could change or be removed at any time
+    without prior notice.
+
 If you like some processes to start automatically, along with the webserver
 (``symfony server:start``), add a configuration file to your project: