[doc]Composer is not a package manager.

It is a issue i found when I translate it.
> Composer is not a package manager, It's a tool for dependency management.

According to their [doc](https://getcomposer.org/doc/00-intro.md)
>Composer is a tool for dependency management in PHP. It allows you to declare the dependent libraries your project needs and it will install them in your project for you.

and 

> **Composer is not a package manager.** Yes, it deals with "packages" or libraries, but it manages > them on a per-project basis, installing them in a directory (e.g. vendor) inside your project. By > default it will never install anything globally. Thus, it is a dependency manager.

So i think, It will be better, if we remain their offical appellation.
Merge pull request #2718 from qiansen1386/patch-2

[doc]Composer is not a package manager.
Update view.php
Update files.php
Update CHANGELOG.md
Merge pull request #2719 from klevron/2712

Fix #2712
CS Fix
Merge pull request #2720 from klevron/2712

Minor CS fix
Add id in diff link. #2712
Update files.php
Merge pull request #2721 from thiagotalma/master

Add id in diff link
code cleanup.
Fixes #2691: Null parameters will not be included in the generated URLs by `UrlManager`
Fixes #2713: using relations defined in behavior may throw exception in lazy loading.
added unit tests for boolean attributes

fixes #1022
Update ActiveFormTest.php
fix letter
Fixed references in the guide
Merge pull request #2727 from Asetss/master

fix letter
Fixes #2734: `FileCache::keyPrefix` defaults to empty string now