[12.x] Adds bool:strict docs (#10622)

* Adds bool:strict docs

* working change

* Update validation.md

---------

Co-authored-by: Taylor Otwell <[email protected]>

Author: peterfox

validation.md

@@ -1346,6 +1346,12 @@ The field under validation must have a size between the given _min_ and _max_ (i
 
 The field under validation must be able to be cast as a boolean. Accepted input are `true`, `false`, `1`, `0`, `"1"`, and `"0"`.
 
+You may use the `strict` parameter to only consider the field valid if its value is `true` or `false`:
+
+```php
+'foo' => 'boolean:strict'
+```
+
 <a name="rule-confirmed"></a>
 #### confirmed