minor symfony#8200 Explained the explicit typing in YAML documents (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes symfony#8200).

Discussion
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Explained the explicit typing in YAML documents

I was going to fix symfony#8173 but I couldn't find the explicit typing explanation anywhere, so I include it here.

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When merging this into 3.4, the `!str` tag must be changed by `!!tag` and we must add this:

```
.. versionadded:: 3.4
    The support of ``!!str`` tag was introduced in Symfony 3.4. In previous
    versions you needed to use the ``!str`` tag.
```

Commits
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aa1b968 Explained the explicit typing in YAML documents

Author: xabbuh

components/yaml/yaml_format.rst

@@ -300,4 +300,22 @@ Comments can be added in YAML by prefixing them with a hash mark (``#``):
     Comments are simply ignored by the YAML parser and do not need to be
     indented according to the current level of nesting in a collection.
 
+Explicit Typing
+---------------
+
+The YAML specification defines some tags to set the type of any data explicitly:
+
+.. code-block:: yaml
+
+    data:
+        # this value is parsed as a float number (it will be 3.0 instead of 3)
+        price: !!float 3
+
+        # this value is parsed as binary data encoded in base64
+        picture: !!binary |
+            R0lGODlhDAAMAIQAAP//9/X
+            17unp5WZmZgAAAOfn515eXv
+            Pz7Y6OjuDg4J+fn5OTk6enp
+            56enmleECcgggoBADs=
+
 .. _YAML: http://yaml.org/