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

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.2+
| Fixed tickets | #2570

Using a filename as argument to Yaml::parse is deprecated as of 2.2 (according to the sources) 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.

Obsoletes PR #2620.