Fix incorrect doc statement about SQL keyword usage

The note states that reserved keywords cannot be used.
This is not true, since
1) not all keywords are implemented by all vendors. For example, "user" is not a keyword in MySQL
2) keywords can be used when they are escaped with backticks

Author: ivanbatic

Resources/doc/index.rst

@@ -173,7 +173,7 @@ start:
 
 .. caution::
 
-    ``user`` is a reserved keyword in SQL so you cannot use it as table name.
+    ``user`` is a reserved keyword in the SQL standard. If you need to use reserved words, surround them with backticks, *e.g.* ``@ORM\Table(name="`user`")``
 
 b) MongoDB User class
 .....................