Clarify channel-specific permissions documentation.

Manage Roles and Manage Channels is renamed to "Manage Permissions" and
"Manage Channel" in the discord UI, clarify which fields in the Permission
object those map to.

discord/permissions.py

@@ -161,7 +161,9 @@ class Permissions:
 
     @property
     def manage_roles(self):
-        """Returns True if a user can manage server roles. This role overrides all other permissions."""
+        """Returns True if a user can manage server roles. This role overrides all other permissions.
+
+        This also corresponds to the "manage permissions" channel-specific override."""
         return self._bit(3)
 
     @manage_roles.setter
@@ -170,7 +172,9 @@ class Permissions:
 
     @property
     def manage_channels(self):
-        """Returns True if a user can edit, delete, or create channels in the server."""
+        """Returns True if a user can edit, delete, or create channels in the server.
+
+        This also corresponds to the "manage channel" channel-specific override."""
         return self._bit(4)
 
     @manage_channels.setter