Document TimeHelper::nice takes $format as param

As per the [TimeHelper::nice source code](https://github.com/cakephp/cakephp/blob/master/lib/Cake/Utility/CakeTime.php#L349), it can take $format as param. 

So Document that in the book.
Added info about new `counterCache` for model saving.
Add docs for new CakeRequest feature.
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Added info about changed API for HtmlHelper::css()
locale updates
Merge pull request #487 from dereuromark/2.4

locale updates
Added info about new FileLog config options
Update docs for validation.
Update docs for new feature change.
Merge branch 'master' into 2.4
Update migration guide for new features.
Clarify migration guide.
Merge branch 'master' into 2.4
Update migration guide for new feature.
CakeNumber currencies
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clarify
Merge pull request #511 from dereuromark/2.4-currencies

CakeNumber currencies
Merge branch 'master' into 2.4