minor #6563 [PhpUnitBridge] Add versionadded directive to clock mocking section (z38)

This PR was merged into the 2.8 branch.

Discussion
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[PhpUnitBridge] Add versionadded directive to clock mocking section

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.8+
| Fixed tickets | -

Commits
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bdddaab [PhpUnitBridge] Add versionadded directive to clock mocking section

Author: wouterj

components/phpunit_bridge.rst

@@ -125,6 +125,9 @@ compatibility reasons.
 Time-sensitive Tests
 ---------------------
 
+.. versionadded:: 2.8
+    Support for clock mocking was introduced in Symfony 2.8.
+
 Use Case
 ~~~~~~~~