[WIP][Valiadtor] - EmailConstraint reference
Use of Sphinx markup
Version added and clarifications
Link moved and heading updated
Typo and link label
Lowercase and link label
CS
Blank line restored
Enhanced Firewall Restrictions docs

Squashed commits:
[2081fc4] Enhanced Firewall Restrictions docs for PR symfony/symfony#10404
[8f065bc] Add redirect to new page
[b42e80f] fixed typos
[cc6b07d] tiny update
After backporting something from master (2.5), removing the 2.5-specific features
Added documentation for the new PropertyAccessor::isReadable() and isWritable() methods
[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