feature #3644 made some small addition about our BC promise and semantic versioning (fabpot)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3644).

Discussion
----------

made some small addition about our BC promise and semantic versioning

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | n/a

Commits
-------

a300510 made some small addition about our BC promise and semantic versioning
Minor tweaks after merging #3644 by @stof and @xabbuh
Fixing syntax error
minor #3720 [#3539] A backport of a sentence - the parts that apply to 2.3 (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
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[#3539] A backport of a sentence - the parts that apply to 2.3

Hi guys!

| Q | A |
|----------------|------|
| Doc fix | no |
| New docs | no |
| Applies to | 2.3 |
| Fixed tickets | #3539 |

This backports the 2.3 parts of a paragraph that was added to 2.5 (master). I like to keep the branches different in *only* different features, so it's easy going into the future to merge things.

Thanks!

Commits
-------

3006120 [#3539] A backport of a sentence - the parts that apply to 2.3
Merge branch '2.3' into 2.4
Merge branch '2.4'

Conflicts:
	components/stopwatch.rst
minor #3722 add "Commands as Services" chapter to the cookbook's map (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
----------

add "Commands as Services" chapter to the cookbook's map

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets |

Commits
-------

3580b7b add "Commands as Services" chapter to the cookbook's map
Merge branch '2.4'
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
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`.