doc: add basic docs for the session manager

doc/pipewire-session-manager.dox

@@ -1,4 +1,42 @@
 /** \page page_session_manager PipeWire Session Manager
 
+The \ref page_daemon is primarily a framework that allows devices and
+applications to exchange data.
+
+It provides the mechanism to do so but the policy deciding which components
+can talk to each other and when is controlled by the session manager.  As
+outlined in \ref page_objects_design, PipeWire provides a media graph
+consistent of Devices, Nodes and Ports. The session manager is the one that
+decides on the links between those elements.
+
+Two prominent session managers currently exist:
+- \ref page_media_session - the reference session manager provided by PipeWire
+- [WirePlumber](https://gitlab.freedesktop.org/pipewire/wireplumber), a
+modular session manager based on GObject
+
+This page describes some of the requirements for session managers in general.
+
+## Client management
+
+PipeWire provides a \ref page_access "permission system" to limit client's
+acccess to resources but only \ref page_module_access "basic permission
+handling". The session manager is expected to decide whether clients may
+access specific resources.
+
+## Device management
+
+PipeWire's responsibility is to open devices, however the decision on which
+devices should be opened is the job of a session manager, including the
+configuration of those devices.
+
+
+## Endpoint grouping
+
+An endpoint is, effectively, a group of Nodes that are a logical unit that can
+consume or produce media data. For example, a Bluetooth speaker may present as
+several Nodes but is only one logical unit to stream audio to.
+
+See \ref page_objects_design for details on Endpoints.
+
 */