Adds docs for the stages package

https://pulp.plan.io/issues/3844
re #3844

docs/plugins/plugin-api/overview.rst

@@ -14,6 +14,7 @@ Plugin API reaches stability with v1.0. For the latest version of the Plugin API
     viewsets
     tasking
     download
+    stages
     changeset
 
 .. automodule:: pulpcore.plugin

docs/plugins/plugin-api/stages.rst

@@ -0,0 +1,66 @@
+.. _stages-docs:
+
+pulpcore.plugin.stages
+======================
+
+Plugin writers can use the Stages API to create a high-performance download-and-saving pipeline
+to make writing sync code easier. There are two parts to the API:
+
+1. DeclarativeVersion is a generic pipeline useful for most synchronization use cases.
+2. The Stages themselves
+
+
+.. _declarative-version:
+
+DeclarativeVersion
+^^^^^^^^^^^^^^^^^^
+
+.. autoclass:: pulpcore.plugin.stages.DeclarativeVersion
+
+.. autoclass:: pulpcore.plugin.stages.FirstStage
+
+.. autoclass:: pulpcore.plugin.stages.DeclarativeArtifact
+
+.. autoclass:: pulpcore.plugin.stages.DeclarativeContent
+
+
+.. _stages-api:
+
+Stages API
+^^^^^^^^^^
+
+.. autofunction:: pulpcore.plugin.stages.create_pipeline
+
+.. autofunction:: pulpcore.plugin.stages.end_stage
+
+
+.. _artifact-stages:
+
+Artifact Related Stages
+^^^^^^^^^^^^^^^^^^^^^^^
+
+.. autoclass:: pulpcore.plugin.stages.artifact_downloader
+
+.. autofunction:: pulpcore.plugin.stages.artifact_saver
+
+.. autofunction:: pulpcore.plugin.stages.query_existing_artifacts
+
+
+.. _content-stages:
+
+Content Related Stages
+^^^^^^^^^^^^^^^^^^^^^^
+
+.. autofunction:: pulpcore.plugin.stages.content_unit_saver
+
+.. autofunction:: pulpcore.plugin.stages.query_existing_content_units
+
+
+.. _content-association-stages:
+
+Content Association and Unassociation Stages
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. autoclass:: pulpcore.plugin.stages.content_unit_association
+
+.. autoclass:: pulpcore.plugin.stages.content_unit_unassociation

plugin/pulpcore/plugin/stages/association_stages.py

@@ -29,7 +29,8 @@ class content_unit_association:
     associated. Since it's a stream the total count isn't known until it's finished.
 
     Args:
-        new_version (RepositoryVersion): The RespositoryVersion this stage associates content with.
+        new_version (pulpcore.plugin.models.RepositoryVersion): The repo version this stage
+            associates content with.
 
     Returns:
         An object containing the content_unit_association stage to be included in a pipeline.
@@ -112,7 +113,8 @@ class content_unit_unassociation:
     un-associated. Since it's a stream the total count isn't known until it's finished.
 
     Args:
-        new_version (RepositoryVersion): The RespositoryVersion this stage unassociates content from
+        new_version (pulpcore.plugin.models.RepositoryVersion): The repo version this stage
+            unassociates content from
 
     Returns:
         An object containing the configured content_unit_unassociation stage to be included in a

plugin/pulpcore/plugin/stages/declarative_version.py

@@ -92,7 +92,8 @@ def __init__(self, first_stage, repository, sync_mode='mirror'):
 
         Args:
              first_stage (FirstStage): The first stage to receive `DeclarativeContent` from
-             repository (Repository): The repository receiving the new version
+             repository (pulpcore.plugin.models.Repository): The repository receiving the new
+                 version
              sync_mode (str): 'mirror' removes content units from the RepositoryVersion that are not
                  queued to DeclarativeVersion. 'additive' only adds content units queued to
                  DeclarativeVersion, and does not remove any pre-existing units in the