feat: add constants for MONTH and YEAR time partitioning types (#283)

Co-authored-by: Steffany Brown <30247553+steffnay@users.noreply.github.com>

Author: tswast

google/cloud/bigquery/table.py

@@ -1980,20 +1980,37 @@ class TimePartitioningType(object):
  HOUR = "HOUR"
  """str: Generates one partition per hour."""
 
+ MONTH = "MONTH"
+ """str: Generates one partition per month."""
+
+ YEAR = "YEAR"
+ """str: Generates one partition per year."""
+
 
 class TimePartitioning(object):
  """Configures time-based partitioning for a table.
 
  Args:
  type_ (Optional[google.cloud.bigquery.table.TimePartitioningType]):
  Specifies the type of time partitioning to perform. Defaults to
- :attr:`~google.cloud.bigquery.table.TimePartitioningType.DAY`,
- which is the only currently supported type.
+ :attr:`~google.cloud.bigquery.table.TimePartitioningType.DAY`.
+
+ Supported values are:
+
+ * :attr:`~google.cloud.bigquery.table.TimePartitioningType.HOUR`
+ * :attr:`~google.cloud.bigquery.table.TimePartitioningType.DAY`
+ * :attr:`~google.cloud.bigquery.table.TimePartitioningType.MONTH`
+ * :attr:`~google.cloud.bigquery.table.TimePartitioningType.YEAR`
+
  field (Optional[str]):
  If set, the table is partitioned by this field. If not set, the
  table is partitioned by pseudo column ``_PARTITIONTIME``. The field
- must be a top-level ``TIMESTAMP`` or ``DATE`` field. Its mode must
- be ``NULLABLE`` or ``REQUIRED``.
+ must be a top-level ``TIMESTAMP``, ``DATETIME``, or ``DATE``
+ field. Its mode must be ``NULLABLE`` or ``REQUIRED``.
+
+ See the `time-unit column-partitioned tables guide
+ <https://cloud.google.com/bigquery/docs/creating-column-partitions>`_
+ in the BigQuery documentation.
  expiration_ms(Optional[int]):
  Number of milliseconds for which to keep the storage for a
  partition.