Docs build in google-cloud-documentai fails with Unexpected section title.

[parthea]

I had to manually fix the docs build to unblock a release PR that had a comment looking like this:

            Currently support query strings are:
            ------------------------------------

            ``SplitType=DATASET_SPLIT_TEST|DATASET_SPLIT_TRAIN|DATASET_SPLIT_UNASSIGNED``

The issue is that the source proto is missing a hyphen for a list item. In addition, there are headings used in the proto comments which are not allowed according to https://google.aip.dev/192#formatting.

https://github.com/googleapis/googleapis/blob/a70dd17d128746c3ce9f3f116fc6404cfdb92f15/google/cloud/documentai/v1beta3/document_service.proto#L301-L307

  // ## Currently support query strings are:
  //
  // `SplitType=DATASET_SPLIT_TEST|DATASET_SPLIT_TRAIN|DATASET_SPLIT_UNASSIGNED`
  // - `LabelingState=DOCUMENT_LABELED|DOCUMENT_UNLABELED|DOCUMENT_AUTO_LABELED`
  // - `DisplayName=\"file_name.pdf\"`
  // - `EntityType=abc/def`
  // - `TagName=\"auto-labeling-running\"|\"sampled\"`

The supported format is

  // Currently support query strings are:
  //
  // - `SplitType=DATASET_SPLIT_TEST|DATASET_SPLIT_TRAIN|DATASET_SPLIT_UNASSIGNED`
  // - `LabelingState=DOCUMENT_LABELED|DOCUMENT_UNLABELED|DOCUMENT_AUTO_LABELED`
  // - `DisplayName=\"file_name.pdf\"`
  // - `EntityType=abc/def`
  // - `TagName=\"auto-labeling-running\"|\"sampled\"`