minor symfony#23821 Fixed UPGRADE-4.0 about Container::set (lyrixx)

This PR was merged into the 3.3 branch.

Discussion
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Fixed UPGRADE-4.0 about Container::set

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | no (doc fix)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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0a61279 Fixed UPGRADE-4.0 about Container::set

Author: nicolas-grekas

UPGRADE-4.0.md

@@ -116,8 +116,8 @@ DependencyInjection
 
  * Using unsupported options to configure service aliases raises an exception.
 
- * Setting or unsetting a private service with the `Container::set()` method is
-   no longer supported. Only public services can be set or unset.
+ * Setting or unsetting a service with the `Container::set()` method is
+   no longer supported. Only synthetic services can be set or unset.
 
  * Checking the existence of a private service with the `Container::has()`
    method is no longer supported and will return `false`.