minor #10055 Mentioned the 767 bytes index limit and its solution (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #10055).

Discussion
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Mentioned the 767 bytes index limit and its solution

This fixes #6398. Sadly this error is very common (I also faced myself several times) and the error message is cryptic, so let's document this problem and its solution.

Commits
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092b7c0 Mentioned the 767 bytes index limit and its solution

Author: javiereguiluz

doctrine.rst

@@ -149,6 +149,15 @@ can automatically generate an empty ``test_project`` database for you:
     support 4-byte unicode characters, and strings containing them will be
     truncated. This is fixed by the `newer utf8mb4 character set`_.
 
+    .. caution::
+
+        MySQL sets a `limit of 767 bytes for the index key prefix`_. When using
+        ``utf8mb4``, string columns with 255 character length surpass that limit.
+        This means that any column of type ``string`` and ``unique=true`` must
+        set its maximum ``length`` to ``190``. Otherwise, you'll see this error:
+        *"[PDOException] SQLSTATE[42000]: Syntax error or access violation:
+        1071 Specified key was too long; max key length is 767 bytes"*.
+
 .. note::
 
     If you want to use SQLite as your database, you need to set the path
@@ -838,3 +847,4 @@ Learn more
 .. _`FrameworkExtraBundle documentation`: https://symfony.com/doc/current/bundles/SensioFrameworkExtraBundle/annotations/converters.html
 .. _`newer utf8mb4 character set`: https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html
 .. _`Transactions and Concurrency`: http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/transactions-and-concurrency.html
+.. _`limit of 767 bytes for the index key prefix`: https://dev.mysql.com/doc/refman/5.6/en/innodb-restrictions.html