minor #4104 Use ${APACHE_LOG_DIR} instead of /var/log/apache2 (xamgreen)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4104).

Discussion
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Use ${APACHE_LOG_DIR} instead of /var/log/apache2

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none
It would be a better solution to use envs in the configfile.

Commits
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b8328c5 APACHE_LOG_DIR note added
8753f37 Use ${APACHE_LOG_DIR} instead of /var/log/apache2

cookbook/configuration/web_server_configuration.rst

@@ -51,7 +51,10 @@ are:
         ErrorLog /var/log/apache2/project_error.log
         CustomLog /var/log/apache2/project_access.log combined
     </VirtualHost>
-
+.. note::
+    You probably want to use ``${APACHE_LOG_DIR}/`` instead of ``/var/log/apache2/`` 
+    for the logging-paths, if your system supports the ``APACHE_LOG_DIR`` variable.
+
 .. note::
 
     For performance reasons, you will probably want to set