Add many docstrings

Closes #111 (closed)

arkindex_worker/worker/__init__.py

 # -*- coding: utf-8 -*-
+"""
+Base classes to implement Arkindex workers.
+"""
+
 import json
 import os
 import sys
@@ -21,10 +25,29 @@ from arkindex_worker.worker.version import WorkerVersionMixin  # noqa: F401
 
 
 class ActivityState(Enum):
+    """
+    Processing state of an element.
+    """
+
     Queued = "queued"
+    """
+    The element has not yet been processed by a worker.
+    """
+
     Started = "started"
+    """
+    The element is being processed by a worker.
+    """
+
     Processed = "processed"
+    """
+    The element has been successfully processed by a worker.
+    """
+
     Error = "error"
+    """
+    An error occurred while processing this element.
+    """
 
 
 class ElementsWorker(
@@ -36,6 +59,13 @@ class ElementsWorker(
     EntityMixin,
     MetaDataMixin,
 ):
+    """
+    Base class for ML workers that operate on Arkindex elements.
+
+    This class inherits from numerous mixin classes found in other modules of
+    ``arkindex.worker``, which provide helpers to read and write to the Arkindex API.
+    """
+
     def __init__(self, description="Arkindex Elements Worker", support_cache=False):
         super().__init__(description, support_cache)
 
@@ -52,11 +82,20 @@ class ElementsWorker(
             nargs="+",
             help="One or more Arkindex element ID",
         )
+
         self.classes = {}
 
         self._worker_version_cache = {}
 
     def list_elements(self):
+        """
+        List the elements to be processed, either from the CLI arguments or
+        the cache database when enabled.
+
+        :return: An iterable of :class:`CachedElement` when cache support is enabled,
+           and a list of strings representing element IDs otherwise.
+        :rtype: Iterable[CachedElement] or List[str]
+        """
         assert not (
             self.args.elements_list and self.args.element
         ), "elements-list and element CLI args shouldn't be both set"
@@ -83,6 +122,12 @@ class ElementsWorker(
 
     @property
     def store_activity(self):
+        """
+        Whether or not WorkerActivity support has been enabled on the DataImport
+        used to run this worker.
+
+        :rtype: bool
+        """
         if self.args.dev:
             return False
         assert (
@@ -99,7 +144,10 @@ class ElementsWorker(
 
     def run(self):
         """
-        Process every elements from the provided list
+        Implements an Arkindex worker that goes through each element returned by
+        :meth:`list_elements`. It calls :meth:`process_element`, catching exceptions
+        and reporting them using the :class:`Reporter`, and handles saving the report
+        once the process is complete as well as WorkerActivity updates when enabled.
         """
         self.configure()
 
@@ -178,13 +226,25 @@ class ElementsWorker(
                 sys.exit(1)
 
     def process_element(self, element):
-        """Override this method to analyze an Arkindex element from the provided list"""
+        """
+        Override this method to implement your worker and process a single Arkindex element at once.
+
+        :param element: The element to process.
+           Will be a CachedElement instance if cache support is enabled,
+           and an Element instance otherwise.
+        :type element: Element or CachedElement
+        """
 
-    def update_activity(self, element_id, state):
+    def update_activity(self, element_id, state) -> bool:
         """
-        Update worker activity for this element
-        Returns False when there is a conflict initializing the activity
-        Otherwise return True or the response payload
+        Update the WorkerActivity for this element and worker.
+
+        :param element_id: ID of the element.
+        :type element_id: str or uuid.UUID
+        :param state ActivityState: New WorkerActivity state for this element.
+        :returns bool: True if the update has been successful or WorkerActivity support is disabled.
+           False if the update has failed due to a conflict; this worker might have already processed
+           this element.
         """
         if not self.store_activity:
             logger.debug(