Clarify settings

Update notes about Symfony Standard
corrections
fix code block indentation
Add additional example for routing integer requirement

This might help for people who are not very good with regular expressions.
Fix getMessage method highlighting
Reformat code block

This will make it more coincide with other code blocks & improve readability.
Fix typo in class name: Appkernel => AppKernel
Fix broken reference link
tweaks to label_attr description
fix file name
Fix namespace typo (remove slash in beginning
Fix coding standards (opening brace on new line)
Fix syntax error in namespace (incorrect slash)
Fix a typo in class url
Update UniqueEntity.rst
[Components][Dependency Injection] fix PHP syntax errors
Merge pull request #3342 from drak/session_c

Clarify settings for native file storage
[#3342] Tweaks thanks to @bicpi and @xabbuh
Merge pull request #3359 from xabbuh/fix-build-errors-2.3

[Components][Form] fix code block indentation
clarify that password is shown only upon submission