It looks like I'll be able to answer my own question. First, I am afraid to say I missed an important note clearly state in the Moodle installation guide. Check it out:
The effect of the previous command is to allow the Apache user account
(www-data in this case) to access and change files within the moodle
site. Many people would consider this a brave move for a new site admin
to implement. In a new moodle you can safely leave this out. A default
Ubuntu install does not have the +a option for the chmod command anyway.
The +a attribute is an ACL (Access Control List) facility which allows
you to set per user access for individual files. For example, OSX has
this by default.
Let me comment on that:
The a+ option of chmod mentioned in bold applies not only for Ubuntu but for others distributions. The important thing is that option as appears in the guide not applies to GNU/Linux. That's for Mac OS X. In GNU/Linux the syntax is something like a+w, ie. Please read to the man chmod for further details. So, basically one can no use the following command -as it is posted- in GNU/Linux:
# chmod -R +a "www-data allow read,delete,write,append,file_inherit,directory_inherit" /path/to/moodle
On the other hand, it is possible to set in GNU/Linux the ACL for /path/to/moodle by issuing:
setfacl -dm "u:wwwrun:rwx" /path/to/moodle
setfacl -dm "g:www:rwx" /path/to/moodle
Change wwwrun by your Apache's user and www for the group.
