Improvements to registering an extension (#2)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #9644 (symfony) and #3231 (symfony-docs)

1. The original text is good but the topic of that paragraph is not "extension conventions" but "registering an extension" (i.e. if you follow these conventions then register goes like so ..)

2. Cleaned up different terminology "Conventions" v "Standards" -> just conventions now

3. Move this part to a more logical place. First create then register, then modify for additional needs.
Changed due to comments in repo collab
typos, CS and PSR links fixed
[security] adding a tip about supported algorithm for 'hash' function
use a phpfunction link + vocabulary change (method -> function)
Change wording of voters
[Book][Validation] validate method in Validator returns ContraintViolationList object not an array
applies WouterJ suggestion
Small fixes
Documented CSRF protection in login forms
Added references
Removed all occurences of $this->getRequest()

The request is a data object, and so the request should not be
inside the container. That's why the request will be removed
from the container in Symfony 3.0

So it is better to let the HttpKernel injects the request when
the user want to use the request.

More over, it is faster. The HttpKernel will always check if the
request should be injected. So injecting the request is time constant.
However, retrieve the request from the container will execute more
calls.
Moved "Impersonating a User" to a cookbook entry
switch custom DQL code example from multi-manager to single
switch custom DQL code example from multi-manager to single (PHP and XML)
Clarify settings

Update notes about Symfony Standard
corrections
[#2362] Documentation for label_attr option in Form Compontent
fix code block syntax
Fixed comments