Add docstrings to Duration properties and in_*/total_* methods

[JazzyJJ0601]

Most of Duration's public properties and methods had no docstring, and a few are easy to misread: weeks only counts the days part (not years/months), seconds is only the time part (not whole days), and the total_* methods count a year as 365 days and a month as 30. This adds a short docstring to each, in the same style as in_words().

Docstrings only; no code changes. Checked against real values, e.g. pendulum.duration(years=1, months=2, days=10, hours=5, minutes=3, seconds=7) has weeks == 1, remaining_days == 3, seconds == 18187, remaining_seconds == 7 and in_weeks() == 62. pytest tests/duration passes.

AI disclosure: I found the gap with an audit tool I'm building, drafted the text with an AI assistant (Claude), and checked every docstring against the code and against real values by hand.

(Audit tool: Legacy Doc-AI.)