Merge pull request #2947 from ChristianGaertner/patch-1

[WCM] Added docs for the changes proposed in symfony/symfony#8908
[#2947] Adding versionadded details
Merge pull request #2960 from WouterJ/doc_php_stopwatch

Documented Stopwatch Helper
[#2960] Minor changes thanks to @xabbuh
Merge pull request #2728 from imobilis/ticket_7884

[WCM] [Process] Doc for output flush methods.
[#2728] Minor tweaks to docs on Process flush methods
Merge pull request #2971 from xabbuh/issue-2958

document the getEncodings() method of the Request class
[#2971] Minor language tweak to be consistent with the other areas

The language used by @xabbuh is actually straight from the PHPDoc, which is cool, but I don't love
the wording there, and having some variety in wording might help clarification.
Merge branch '2.3'
fix versionadded directive
[#2728] Fixing typo thanks to @xabbuh
Merge pull request #3036 from xabbuh/fix-build-errors-master

fix versionadded directive
Merge branch '2.3'

Conflicts:
	reference/constraints/Image.rst
Merge pull request #3021 from fabpot/request-factory

added documentation about Request::setFactory()
[#3021] Minor tweaks thanks to @WouterJ and @GromNaN
Creates Expression Language chapter
Bootstraps intro
use literal for class name
Added operators
Added another operator