Fixed link to Doctrine manual.

Author: dlsniper

book/doctrine.rst

@@ -21,7 +21,7 @@ be.
 
 
  Doctrine doesn't auto-escape reserved SQL keywords, for further reference
- check Doctrine manual.
+ check Doctrine manual, `Reserved SQL keywords documentation`_.
 
 
  You can also persist data to `MongoDB`_ using Doctrine ODM library. For