minor #6616 Better explain the mandatory/convention location of some elements (rcousens, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #6616).

Discussion
----------

Better explain the mandatory/convention location of some elements

This finishes #5559 by implementing the comments made by @wouterj

Commits
-------

eee4750 Fixed some wrong explanations
4a2c92f Tweaked the table contents
cd0b889 Fix spacing on third column
b853e52 Fix as per comments
99da972 Add third column to specific emplacements table describing relationship

Author: wouterj

cookbook/bundles/best_practices.rst

@@ -113,22 +113,23 @@ them under the ``cache/`` or ``log/`` directory of the host application. Tools
 can generate files in the bundle directory structure, but only if the generated
 files are going to be part of the repository.
 
-The following classes and files have specific emplacements:
-
-===============================  =============================
-Type                             Directory
-===============================  =============================
-Commands                         ``Command/``
-Controllers                      ``Controller/``
-Service Container Extensions     ``DependencyInjection/``
-Event Listeners                  ``EventListener/``
-Model classes [1]                ``Model/``
-Configuration                    ``Resources/config/``
-Web Resources (CSS, JS, images)  ``Resources/public/``
-Translation files                ``Resources/translations/``
-Templates                        ``Resources/views/``
-Unit and Functional Tests        ``Tests/``
-===============================  =============================
+The following classes and files have specific emplacements (some are mandatory
+and others are just conventions followed by most developers):
+
+===============================  =============================  ================
+Type                             Directory                      Mandatory?
+===============================  =============================  ================
+Commands                         ``Command/``                   Yes
+Controllers                      ``Controller/``                No
+Service Container Extensions     ``DependencyInjection/``       Yes
+Event Listeners                  ``EventListener/``             No
+Model classes [1]                ``Model/``                     No
+Configuration                    ``Resources/config/``          No
+Web Resources (CSS, JS, images)  ``Resources/public/``          Yes
+Translation files                ``Resources/translations/``    Yes
+Templates                        ``Resources/views/``           Yes
+Unit and Functional Tests        ``Tests/``                     No
+===============================  =============================  ================
 
 [1] See :doc:`/cookbook/doctrine/mapping_model_classes` for how to handle the
 mapping with a compiler pass.