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
Merge pull request #3224 from peterkokot/patch-1

Typos
Merge pull request #3236 from flip111/patch-2

Improvements to registering an extension (#2)
[#3236] minor tweaks
Merge branch '2.2' into 2.3

Conflicts:
	cookbook/security/entity_provider.rst
	reference/forms/types/options/disabled.rst.inc
Changed wording of roles section

- Changed wording to make it more clear that any one of the required
  roles for a given resource will allow access to that resource.