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

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #2570

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.