docs: add deprecation comments for message fields and enums

[parthea]

Fixes #1733 🦕

This PR is on hold until the client library AIPs are updated: https://google.aip.dev/client-libraries/4210 . Also see aip-dev/google.aip.dev#1198

[parthea]

I tested the changes locally by regenerating the python client for google-cloud-contentwarehouse with these fixes. I've confirmed that the docs build passes. See the diff below:

(py39) partheniou@partheniou-vm-3:~/git/google-cloud-python$ git diff
diff --git a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/common.py b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/common.py
index 9aa4f3ee0a..0ddb949c62 100644
--- a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/common.py
+++ b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/common.py
@@ -80,6 +80,7 @@ class DatabaseType(proto.Enum):
         DB_INFRA_SPANNER (1):
             Internal Spanner
         DB_CLOUD_SQL_POSTGRES (2):
+            Deprecated.
             Cloud Sql with a Postgres Sql instance
     """
     DB_UNKNOWN = 0
diff --git a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document.py b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document.py
index 7f4f054f8e..31c2914ca1 100644
--- a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document.py
+++ b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document.py
@@ -141,6 +141,7 @@ class Document(proto.Message):
 
             This field is a member of `oneof`_ ``structured_content``.
         structured_content_uri (str):
+            Deprecated.
             A path linked to structured content file.
         raw_document_path (str):
             Raw document file in Cloud Storage path.
@@ -166,12 +167,14 @@ class Document(proto.Message):
             the byte representation of a PDF file, then this should be
             set to: RAW_DOCUMENT_FILE_TYPE_PDF.
         async_enabled (bool):
+            Deprecated.
             If true, makes the document visible to
             asynchronous policies and rules.
         content_category (google.cloud.contentwarehouse_v1.types.ContentCategory):
             Indicates the category (image, audio, video
             etc.) of the original content.
         text_extraction_disabled (bool):
+            Deprecated.
             If true, text extraction will not be
             performed.
         text_extraction_enabled (bool):
diff --git a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document_link_service.py b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document_link_service.py
index f2cc3e38f9..ca260a8faf 100644
--- a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document_link_service.py
+++ b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/document_link_service.py
@@ -127,6 +127,7 @@ class ListLinkedSourcesRequest(proto.Message):
             The maximum number of document-links to
             return. The service may return fewer than this
             value.
+
             If unspecified, at most 50 document-links will
             be returned. The maximum value is 1000; values
             above 1000 will be coerced to 1000.
diff --git a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/filters.py b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/filters.py
index 369e3b2f03..39637e3f3f 100644
--- a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/filters.py
+++ b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/filters.py
@@ -94,6 +94,7 @@ class DocumentQuery(proto.Message):
             must be set. In addition, all other input fields related to
             search (pagination, histograms, etc.) will be ignored.
         custom_property_filter (str):
+            Deprecated.
             This filter specifies a structured syntax to match against
             the [PropertyDefinition].[is_filterable][] marked as
             ``true``. The syntax for this expression is a subset of SQL
diff --git a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/histogram.py b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/histogram.py
index 0635422b2c..d30c267381 100644
--- a/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/histogram.py
+++ b/packages/google-cloud-contentwarehouse/google/cloud/contentwarehouse_v1/types/histogram.py
@@ -44,6 +44,7 @@ class HistogramQuery(proto.Message):
             Controls if the histogram query requires the
             return of a precise count. Enable this flag may
             adversely impact performance.
+
             Defaults to true.
         filters (google.cloud.contentwarehouse_v1.types.HistogramQueryPropertyNameFilter):
             Optional. Filter the result of histogram