[DOC] Docstring coverage in Core

[karlhigley]

Modules missing docstrings:

• core
• dag
• io
• schema

Name	Total	Miss	Cover	Cover%
core/dispatch.py	44	22	22	50%
core/utils.py	13	9	4	31%
core/worker.py	3	1	2	67%
dag/base_operator.py	9	4	5	56%
dag/graph.py	4	3	1	25%
dag/node.py	12	11	1	8%
dag/selector.py	3	1	2	67%
dag/ops/concat_columns.py	4	3	1	25%
dag/ops/identity.py	2	1	1	50%
dag/ops/selection.py	4	4	0	0%
dag/ops/subset_columns.py	2	1	1	50%
dag/ops/subtraction.py	4	4	0	0%
io/avro.py	6	5	1	17%
io/csv.py	4	3	1	25%
io/dataframe_engine.py	4	3	1	25%
io/dataframe_iter.py	1	1	0	0%
io/dataset.py	14	2	12	86%
io/dataset_engine.py	7	5	2	29%
io/hugectr.py	1	1	0	0%
io/parquet.py	22	16	6	27%
io/shuffle.py	2	1	1	50%
io/writer.py	13	13	0	0%
io/writer_factory.py	1	1	0	0%
schema/schema.py	15	2	13	87%
schema/io/schema_bp.py	51	11	40	78%
TOTAL	286	128	158	55.2%
(5 of 30 files omitted due to complete coverage)