Update docstrings on entity type

Describe the instance variables and there types with
docstrings and comments and such, so that it is more
known what this type is for.

Also removes docstring saying this is only for jobs
and jobboards since it is really a more generic entity
class and has no direct tie to jobboards.

Change-Id: I98adb161adcb4ce096c1ee0e6c3377eb71383a90

taskflow/types/entity.py

@@ -16,9 +16,19 @@
 
 
 class Entity(object):
-    """Entity object(s) to be registered on jobboard.
+    """Entity object that identifies some resource/item/other.
 
-    Now only supports 'kind' of 'conductor'.
+    :ivar kind: **immutable** type/kind that identifies this
+                entity (typically unique to a library/application)
+    :type kind: string
+    :ivar name: **immutable** name that can be used to uniquely
+                identify this entity among many other entities
+    :type name: string
+    :ivar metadata: **immutable** dictionary of metadata that is
+                    associated with this entity (and typically
+                    has keys/values that further describe this
+                    entity)
+    :type metadata: dict
     """
     def __init__(self, kind, name, metadata):
         self.kind = kind