minor #4208 Added a note about the lacking features of Yaml Component…

… (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added a note about the lacking features of Yaml Component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

In `symfony/symfony` repository there are a lot of issues reporting errors of the Yaml Component. These aren't errors but lacking features of the Yaml Component.

This PR tries to clearly state that Yaml Component lacks lots of features on purpose and provides a simple list of the most notable missing features.

Commits
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fe45194 Added a note about the lacking features of Yaml Component

components/yaml/introduction.rst

@@ -18,8 +18,8 @@ standard for all programming languages. YAML is a great format for your
 configuration files. YAML files are as expressive as XML files and as readable
 as INI files.
 
-The Symfony2 Yaml component implements the YAML 1.2 version of the
-specification.
+The Symfony2 Yaml Component implements a selected subset of features defined in
+the `YAML 1.2 version specification`_.
 
 .. tip::
 
@@ -40,9 +40,10 @@ Why?
 Fast
 ~~~~
 
-One of the goals of the Symfony Yaml component is to find the right balance
-between speed and features. It supports just the needed features to handle
-configuration files.
+One of the goals of Symfony Yaml is to find the right balance between speed and
+features. It supports just the needed features to handle configuration files.
+Notable lacking features are: document directives, multi-line quoted messages,
+compact block collections and multi-document files.
 
 Real Parser
 ~~~~~~~~~~~
@@ -164,7 +165,7 @@ array to its YAML representation:
 
 .. note::
 
-    Of course, the Symfony2 YAML dumper is not able to dump resources. Also,
+    Of course, the Symfony2 Yaml dumper is not able to dump resources. Also,
     even if the dumper is able to dump PHP objects, it is considered to be a
     not supported feature.
 
@@ -214,3 +215,4 @@ representation to the inline one:
 
 .. _YAML: http://yaml.org/
 .. _Packagist: https://packagist.org/packages/symfony/yaml
+.. _`YAML 1.2 version specification`: http://yaml.org/spec/1.2/spec.html