Minor tweaks related to #3368
Minor tweaks after merging #3644 by @stof and @xabbuh
Fixing syntax error
[book] [validation] fixed minor typos
[book] [testing] fixed the path of the phpunit.xml file

Sometimes is referenced as phpunit.xml and other times as
app/phpunit.xml I think that we should always use the explicit
app/phpunit.xml path, except when explaining the phpunit.xml
file in general.
[book] [controller] fixed the code of a session sample code

This code was updated in the past to use the $request controller
injection and the resulting code was a bit confusing. When you get
the attribute set by another controller, it's better to use a
different attribute name, to make it clear that it wasn't set at
this controller.
[book] [routing] used the American term "bidirectional"
instead of the British bi-directional
[book] [routing] reworded the note about generating URLs in console
[book] [routing] fixed a note that wasn't properly updated

Previously, the code used the generate() method of the service and
the note explained the generateUrl() method of the base controller.
Then, the example was updated to use the generateUrl(), but the
note kept explaining the generateUrl(). This change is just about
explaining in the note the alterantive generate() method of the service.
[book] [forms] misc. fixes and tweaks
[book] [forms] minor fixes and rewordings
[book] [routing] minor rewording and fixed some sample code
correct RuntimeException reference
Update security.rst

I'm not a native speaker but I think that it sounds better by adding that before the word `start`.
Improved the sentence
Added a note about inlined private services
Emphasized the fact that this only applies when the private service is used as argument for a single service
added the title of a cross reference
Update routing.rst. Explain using url() v. path().
Fixed event listeners priority