[SPARK-57740][PYTHON] Fix unix_micros truncation disclaimer and add unix_nanos cross-references to unix_* See Also sections

[jubins]

What is the purpose of the change

Fixes SPARK-57740 — two related doc-correctness issues in the unix_* epoch-unit function family in python/pyspark/sql/functions/builtin.py:

1. Missing truncation disclaimer in unix_micros. unix_millis and unix_seconds both include "Truncates higher levels of precision." in their one-line description. unix_micros did not, despite silently dropping sub-microsecond digits when given a nanosecond-precision input (TIMESTAMP_LTZ(p) / TIMESTAMP_NTZ(p), p in [7, 9]). This became a practical concern once unix_nanos (SPARK-57579) made nanosecond-precision timestamps usable in PySpark.

2. Missing unix_nanos cross-references. unix_nanos (SPARK-57579) correctly links to all three siblings in its See Also section. None of the three sibling functions were updated to link back to unix_nanos, breaking discoverability of the function family from the existing functions.

Brief change log

• python/pyspark/sql/functions/builtin.py:
  • unix_micros: added "Truncates higher levels of precision." to the one-line description
  • unix_micros, unix_millis, unix_seconds: added :meth:\pyspark.sql.functions.unix_nanos`to eachSee Also` section

Verifying this change

These are documentation-only changes with no logic impact. Verified by inspection that:

• All four unix_* functions now consistently include a truncation disclaimer in their description
• All four unix_* functions now cross-reference each other in their See Also sections symmetrically

Does this pull request potentially affect one of the following parts

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public/@Evolving: no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: no
• The S3 file system connector: no

Documentation

Does this pull request introduce a new feature? No — documentation fix only.

Was generative AI tooling used to co-author this PR?

• Yes — Claude Code was used as a pair-programming assistant. All code was written, understood, and verified by the author.
  Generated-by: Claude Opus 4.8

[MaxGekk]

@jubins please, make PR's title concise