feature #7199 Documented the compiler passes priorities (javiereguiluz)

This PR was squashed before being merged into the 3.2 branch (closes #7199).

Discussion
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Documented the compiler passes priorities

This fixes #6671.

Commits
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6c96d38 Documented the compiler passes priorities

Author: xabbuh

components/dependency_injection/compilation.rst

@@ -423,6 +423,23 @@ been run, use::
         PassConfig::TYPE_AFTER_REMOVING
     );
 
+.. versionadded:: 3.2
+    The option to prioritize compiler passes was added in Symfony 3.2.
+
+You can also control the order in which compiler passes are run for each
+compilation phase. Use the optional third argument of ``addCompilerPass()`` to
+set the priority as an integer number. The default priority is ``0`` and the higher
+its value, the earlier it's executed::
+
+    // ...
+    // FirstPass is executed after SecondPass because its priority is lower
+    $container->addCompilerPass(
+        new FirstPass(), PassConfig::TYPE_AFTER_REMOVING, 10
+    );
+    $container->addCompilerPass(
+        new SecondPass(), PassConfig::TYPE_AFTER_REMOVING, 30
+    );
+
 .. _components-dependency-injection-dumping:
 
 Dumping the Configuration for Performance