[skip ci] French start-gii.md

related to #5083

[skip ci] Expand abnormally used ternary operator to if

[skip ci] French start-databases.md

[skip ci] bootstrapping docs translated

grammar fix [ci skip]

[ci skip] Update rest-response-formatting.md

[skip ci] Update rest-routing.md

fix typo

Added a note that pluralization of controller IDs in RESTful endpoints by yii\rest\UrlRule doesn't always simply add "s" to the end of the controller ID.  I cited an example of controllers whose IDs end in the letter "x", for example "BoxController", whose RESTful endpoints are pluralized to "boxes"

close #5261

fixes #5260

[ci skip] Update start-databases.md

[skip ci] Update start-forms.md

[skip ci] Update start-installation.md

[skip ci] Update documentation to mention addFlash

Fixes #5232 and #5231

related to #5128