Document YAML callable error handling

[sanjaydilli]

Summary

• Documents how YAML %CALLABLE positional and keyword arguments are selected.
• Describes compatibility with legacy comma-separated and raw-string arguments.
• Clarifies that a callable is invoked only once after selecting a compatible argument form.
• Documents that callable exceptions are preserved and are not treated as parsing failures.

Validation

rst2pseudoxml --halt=warning docs/utilities/yaml_markup.rst