minor #1611 [doc] make:entity requires Doctrine (alexislefebvre)

This PR was squashed before being merged into the 1.x-dev branch.

Discussion
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[doc] make:entity requires Doctrine

Someone on Stack Overflow asked (question is not published yet) if `make:entity` can be used without Doctrine.

It looks like MakerBundle can only manage Doctrine entities, so I added it in the docs, hopefully that is true. Otherwise we'll have to change or close this PR.

Commits
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6adfa32 [doc] make:entity requires Doctrine

docs/index.rst

@@ -41,6 +41,9 @@ optional arguments and options. Check them out with the ``--help`` option:
 
     $ php bin/console make:controller --help
 
+.. caution::
+
+    ``make:entity`` requires ``doctrine/orm`` to be installed and configured. This maker support only ORM, not ODM.
 
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