Fix typo in input-validation.md guide [skip ci]

made it consistent with the GD function.

See also https://github.com/yiisoft/yii/pull/3406

Minor changes in Estonian translation

Spanish guide structure-controllers.md [skip ci]

English guide typo [skip ci]

guide mistake: suppose to be "role", not "name" I think.

I think it is a small mistake.

Fix html markdown for span class="not-set" when time is not set

Translation review readme.md

See #4000

Update from remote

Create caching-overview.md

Add translate for caching-overview.md

Added more unit tests to widgets\Breadcrumbs and increased coverage to 100%

Update ActiveForm.php

Incorrect character for generating docs. See http://www.yiiframework.com/doc-2.0/yii-bootstrap-activeform.html

add guide-ru/rest-quick-start.md

Fixes #3979: Alert widget in advanced app is now able to display multiple flash messages of the same type