docs: reference BigQuery REST API defaults in LoadJobConfig descrip… (#1132)

…tion

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes internal issue 213442215 🦕

Author: tswast

google/cloud/bigquery/job/load.py

@@ -33,9 +33,21 @@
 class LoadJobConfig(_JobConfig):
     """Configuration options for load jobs.
 
-    All properties in this class are optional. Values which are :data:`None` ->
-    server defaults. Set properties on the constructed configuration by using
-    the property name as the name of a keyword argument.
+    Set properties on the constructed configuration by using the property name
+    as the name of a keyword argument. Values which are unset or :data:`None`
+    use the BigQuery REST API default values. See the `BigQuery REST API
+    reference documentation
+    <https://cloud.google.com/bigquery/docs/reference/rest/v2/Job#JobConfigurationLoad>`_
+    for a list of default values.
+
+    Required options differ based on the
+    :attr:`~google.cloud.bigquery.job.LoadJobConfig.source_format` value.
+    For example, the BigQuery API's default value for
+    :attr:`~google.cloud.bigquery.job.LoadJobConfig.source_format` is ``"CSV"``.
+    When loading a CSV file, either
+    :attr:`~google.cloud.bigquery.job.LoadJobConfig.schema` must be set or
+    :attr:`~google.cloud.bigquery.job.LoadJobConfig.autodetect` must be set to
+    :data:`True`.
     """
 
     def __init__(self, **kwargs):