Update yaml component docs to reflect 2.3 Yaml::parse(filename) deprecation

Using a filename as argument to Yaml::parse is deprecated as of 2.3 and will be removed from sf 3.0. Updated the documentation to indicate this, and include the warning from the source about the need to validate the input first.

Conflicts:

	components/yaml/introduction.rst
Changed note to caution; reworded; changed deprecation version

Looking at the Symfony code, this was actually deprecated in 2.2. Move the warning text to the front because the validation warning is more important than the deprecation notice.