docs, privileges: move general overview of IRC privileges to doc page

The history and brief overview of how IRC privileges work is not part of
the module/class documentation. It's more appropriate to explain in the
doc page about privileges as a general concept.

docs/source/plugin/privileges.rst

@@ -13,6 +13,29 @@ This will give both OP and Voice privileges to the user named "Nickname" in the
 message it registers and updates its knowledge of a user's privileges in a
 channel, which can be used by plugins in various ways.
 
+Historically, these two privilege levels ("op" and "voiced") were the only
+channel privileges available:
+
+* :attr:`~sopel.privileges.AccessLevel.OP`: channel operator, set and unset by
+  modes ``+o`` and ``-o``
+* :attr:`~sopel.privileges.AccessLevel.VOICE`: the privilege to send messages to
+  a channel with the ``+m`` mode, set and unset by modes ``+v`` and ``-v``
+
+Since then, other privileges have been adopted by IRC servers and clients:
+
+* :attr:`~sopel.privileges.AccessLevel.HALFOP`: intermediate level between VOICE
+  and OP, set and unset by modes ``+h`` and ``-h``
+* :attr:`~sopel.privileges.AccessLevel.ADMIN`: channel admin, above OP and below
+  OWNER, set and unset by modes ``+a`` and ``-a``
+* :attr:`~sopel.privileges.AccessLevel.OWNER`: channel owner, above ADMIN and OP,
+  set and unset by modes ``+q`` and ``-q``
+
+.. important::
+
+    Not all IRC networks support these added privilege modes. If you are writing
+    a plugin for public distribution, ensure your code behaves sensibly if only
+    ``+v`` (voice) and ``+o`` (op) modes exist.
+
 Access rights
 =============
 

sopel/privileges.py

@@ -13,31 +13,6 @@
 class AccessLevel(enum.IntFlag):
     """Enumeration of available user privilege levels.
 
-    Privilege levels
-    ================
-
-    Historically, there were two user privilege levels in a channel:
-
-    * :data:`~AccessLevel.OP`: channel operator, set and unset by modes ``+o``
-      and ``-o``
-    * :data:`~AccessLevel.VOICE`: the privilege to send messages to a channel
-      with the ``+m`` mode, set and unset by modes ``+v`` and ``-v``
-
-    Since then, other privileges have been adopted by IRC servers and clients:
-
-    * :data:`~AccessLevel.HALFOP`: intermediate level between VOICE and OP, set
-      and unset by modes ``+h`` and ``-h``
-    * :data:`~AccessLevel.ADMIN`: channel admin, above OP and below OWNER, set
-      and unset by modes ``+a`` and ``-a``
-    * :data:`~AccessLevel.OWNER`: channel owner, above ADMIN and OP, set and
-      unset by modes ``+q`` and ``-q``
-
-    .. important::
-
-        Not all IRC networks support these added privilege modes. If you are
-        writing a plugin for public distribution, ensure your code behaves
-        sensibly if only +v (voice) and +o (op) modes exist.
-
     Comparing privileges
     ====================