- Clarify depth-aware short-circuit comment to unambiguously describe
the relationship between current depth and previous validation depth
- Add comment to MappingValueNode case explaining intentional depth+2
behavior from parent MappingNode perspective
- Restructure unmarshalYAMLWithDepthLimit doc comment as bullet list
covering all three safety checks (depth, multi-doc, strict fields)
- Replace t.Error with t.Fatal in TestYAMLEmptyFileRejection to remove
redundant nil guard on subsequent err.Error() call