[SPARK-57742][PYTHON] Add truncation disclaimer to unix_micros docstring

[jubins]

What is the purpose of the change

Fixes SPARK-57742 — adds "Truncates higher levels of precision." to the one-line description of unix_micros, making it consistent with unix_millis and unix_seconds which already carry this disclaimer.

This became a practical gap once unix_nanos (SPARK-57579) made nanosecond-precision timestamps (TIMESTAMP_LTZ(9) / TIMESTAMP_NTZ(9)) usable: a user passing such a column to unix_micros has no indication from the docstring that sub-microsecond digits will be silently dropped.

Brief change log

• python/pyspark/sql/functions/builtin.py: appended "Truncates higher levels of precision." to the one-line summary of unix_micros, matching unix_millis and unix_seconds

Verifying this change

This is a documentation-only change. No behaviour is altered.

• Confirm unix_micros docstring now reads: "Returns the number of microseconds since 1970-01-01 00:00:00 UTC. Truncates higher levels of precision."
• Confirm unix_millis and unix_seconds already carry the same sentence (consistency check)

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 — docstring only
• 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 — this is a docstring fix.

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

• Yes — Claude Code was used as a pair-programming assistant. All changes were reviewed and verified by the author. Generated-by: Claude Opus 4.8