minor #9193 Explain that doctrine:mapping:import will be removed soon (javiereguiluz)

This PR was squashed before being merged into the 4.0 branch (closes #9193).

Discussion
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Explain that doctrine:mapping:import will be removed soon

This fixes #9172.

Commits
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ce9ce73 Explain that doctrine:mapping:import will be removed soon

Author: javiereguiluz

doctrine/reverse_engineering.rst

@@ -4,6 +4,14 @@
 How to Generate Entities from an Existing Database
 ==================================================
 
+.. caution::
+
+    The feature explained in this article doesn't work in modern Symfony
+    applications that have no bundles. The workaround is to temporarily create
+    a bundle. See `doctrine/doctrine#729`_ for details. Moreover, this feature
+    to generate entities from existing databases will be completely removed in
+    the next Doctrine version.
+
 When starting work on a brand new project that uses a database, two different
 situations comes naturally. In most cases, the database model is designed
 and built from scratch. Sometimes, however, you'll start with an existing and
@@ -54,12 +62,6 @@ is to ask Doctrine to introspect the database and generate the corresponding
 metadata files. Metadata files describe the entity class to generate based on
 table fields.
 
-.. caution::
-
-    If your application has *no* bundles, then this command will currently fail!
-    The workaround, is to temporarily create a bundle. See `doctrine/doctrine#729`_
-    for details.
-
 .. code-block:: terminal
 
     $ php bin/console doctrine:mapping:import --force AppBundle xml