Update book/forms.rst

Fix typo (missing ";") in source code sample for dynamic validation groups
Added some standards
[Doc] add link to Varnish website
fix bash command display in components/console/usage
Creates Expression Language chapter
Bootstraps intro
Added operators
Added another operator
Register Commands in the Service Container
[Form] Add a 'submitted' attribute to the form view
Documents Output::*Verbose methods
Update typo in use_empty_data.rst
Fixed typo in direct_submit.rst
[OptionsResolver] Fix syntax errors in code examples (missing in_array closing slashes)

This pull request fixes 2 syntax errors when using `in_array()` in conditions (missing the last closing slash).

It also adds a missing `use` statement in one code example.
Fix typo
make target name unique that is used as a reference
remove wrong notice on locales in the session, add reference to the relevant cookbook article instead
Merge branch 'patch-2' of github.com:nicolas-bastien/symfony-docs into nicolas-bastien-patch-2

Conflicts:
	cookbook/cache/varnish.rst
Merge pull request #3029 from egeloen/f-valid-form-view

[Form] Add a 'submitted' attribute to the form view
Merge pull request #3031 from lyrixx/command-in-the-DIC

[WCM]Register Commands in the Service Container