Replaced backslashes with forward slashes in template path example [skip ci]

Backslash after alias causes 'Invalid path alias' exception in `\yii\BaseYii::getAlias`
Fixes #4600
Merge pull request #4601 from SDKiller/patch-1

Replaced backslashes with forward slashes in template path example [skip...
Update docs for clientValidateAttribute()
Merge pull request #4603 from Skysplit/patch-1

Update docs for clientValidateAttribute()
Fixes #4591: `yii\helpers\Url::to()` will no longer prefix relative URLs with the base URL
Merge branch 'master' of git://github.com/yiisoft/yii2
fixed random rbac testing failure.
Fixed traslation mistake
Fixed test break.
Fixes #4606: doc fix. [skip ci]
re-enabled app tests.
Merge pull request #4604 from pana1990/master

Fixed traslation mistake
Fixes #4609. Fixes #4610.
fixed link
Fixes #4611.
Merge pull request #4618 from xBazilio/patch-4

fixed link
Added `yii\base\InvalidValueException`.
Refactored `yii\web\User`.
Fixed code break.
fixed comment in code snippet