feature #5207 Updated the cookbook about Composer installation (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

Updated the cookbook about Composer installation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Commits
-------

d677c96 Updated the cookbook about Composer installation

Author: wouterj

cookbook/composer.rst

@@ -4,8 +4,12 @@
 Installing Composer
 ===================
 
-`Composer`_ is the package manager used by modern PHP applications and the
-recommended way to install Symfony2.
+`Composer`_ is the package manager used by modern PHP applications. Use Composer
+to manage dependencies in your Symfony applications and to install Symfony Components
+in your PHP projects.
+
+It's recommended to install Composer globally in your system as explained in the
+following sections.
 
 Install Composer on Linux and Mac OS X
 --------------------------------------
@@ -37,8 +41,8 @@ the instructions.
 Learn more
 ----------
 
-You can read more about Composer in `its documentation`_.
+Read the `Composer documentation`_ to learn more about its usage and features.
 
 .. _`Composer`: https://getcomposer.org/
 .. _`getcomposer.org/download`: https://getcomposer.org/download
-.. _`its documentation`: https://getcomposer.org/doc/00-intro.md
+.. _`Composer documentation`: https://getcomposer.org/doc/00-intro.md