minor #13383 [Translation] Documented the debug:translation exit codes (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #13383).

Discussion
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[Translation] Documented the debug:translation exit codes

Fixes #13019.

Commits
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9fe3193 [Translation] Documented the debug:translation exit codes

Author: javiereguiluz

translation/debug.rst

@@ -180,3 +180,33 @@ unused or only the missing messages, by using the ``--only-unused`` or
 
     $ php bin/console debug:translation en --only-unused
     $ php bin/console debug:translation en --only-missing
+
+Debug Command Exit Codes
+------------------------
+
+The exit code of the ``debug:translation`` command changes depending on the
+status of the translations. Use the following public constants to check it::
+
+    use Symfony\Bundle\FrameworkBundle\Command\TranslationDebugCommand;
+
+    // generic failure (e.g. there are no translations)
+    TranslationDebugCommand::EXIT_CODE_GENERAL_ERROR;
+
+    // there are missing translations
+    TranslationDebugCommand::EXIT_CODE_MISSING;
+
+    // there are unused translations
+    TranslationDebugCommand::EXIT_CODE_UNUSED;
+
+    // some translations are using the fallback translation
+    TranslationDebugCommand::EXIT_CODE_FALLBACK;
+
+These constants are defined as "bit masks", so you can combine them as follows::
+
+    if (TranslationDebugCommand::EXIT_CODE_MISSING | TranslationDebugCommand::EXIT_CODE_UNUSED) {
+        // ... there are missing and/or unused translations
+    }
+
+.. versionadded:: 5.1
+
+    The exit codes were introduced in Symfony 5.1