Update parameter docstrings to "string or iterable of strings"

Copilot noted the public docstrings still described multi-value string
filters as "string or list of strings" while the annotations were widened
to `str | Iterable[str] | None`. The user-facing docs now lie about the
contract — pd.Series / np.ndarray / generators / tuples all work at runtime
but read as unsupported.

Replaced 165 occurrences across api.py. The 11 `properties` parameter
docstrings remain "string or list of strings" because that parameter is
intentionally typed `str | list[str] | None` (the comma-join site requires
a list after the single-string wrap).

Author: thodson-usgs