docs(py_utils): add missing docstrings to 5 public utility functions

[RavSinghChandan]

What does this PR do?

Adds missing docstrings to five public utility functions in src/datasets/utils/py_utils.py. Each docstring follows the HuggingFace documentation style with Args, Returns, Raises, and Example sections.

Functions documented:

Function	What it does
glob_pattern_to_regex	Converts a glob pattern to a regex string
has_sufficient_disk_space	Checks whether a directory has enough free space
copyfunc	Returns a shallow copy of a Python function
iflatmap_unordered	Parallel flatmap yielding results in completion order
iter_batched	Iterates over an iterable in fixed-size batches

Motivation

These functions are used throughout the library but lacked any documentation, making it harder for contributors and users to understand their behaviour without reading the implementation.

Tests

No logic changed — documentation only. Existing test suite covers these functions.

ruff check and ruff format --check both pass with no errors.