add missing docstrings to print_writeout and get_sample_size in evaluator_utils.py

[RavSinghChandan]

print_writeout and get_sample_size in lm_eval/evaluator_utils.py are public functions used during evaluation but have no docstrings, making their purpose and expected inputs unclear without reading the implementation.

• print_writeout – logs the context prompt and request string for the first document in a task, useful for debugging.
• get_sample_size – resolves a fractional or absolute sample limit into a concrete document count.

Added Google-style docstrings with Args and Returns sections matching the style used for find_test_root and run_task_tests already in this file.

No logic changes.

[CLAassistant]

All committers have signed the CLA.

[RavSinghChandan]

@googlebot I signed it!