doc/sphinx: Add some documentation about timeline points

Use doxygen ingroup command as to show the symbols in the sphinx
documentation.

Include some basic comments and document the exported functions from
timeline.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

doc/sphinx/toc/libweston/log.rst

@@ -65,6 +65,7 @@ Weston has a few scopes worth mentioning:
 - **xwm-wm-x11** - a scope for the X11 window manager in Weston for supporting
   Xwayland, printing some X11 protocol actions.
 - **content-protection-debug** - scope for debugging HDCP issues.
+- **timeline** - see more at :ref:`timeline points`
 
 .. note::
 
@@ -156,6 +157,38 @@ client connects a new subscriber will be created.  For each stream subscribed a
 subscription will be created.  Enabling the debug-protocol happens using the
 :samp:`--debug` command line.
 
+Timeline points
+---------------
+
+A special log scope is the 'timeline' scope which, together with
+`wesgr <https://github.com/ppaalanen/wesgr>`_ tool, helps diagnose latency issues.
+Timeline points write to this 'timeline' scope in different parts of the
+compositor, including the GL renderer.
+
+As with all other scopes this scope is available over the debug protocol, or by
+using the others :ref:`subscribers`. By far the easiest way to get data out
+of this scope would be to use the debug protocol.
+Then use `wesgr <https://github.com/ppaalanen/wesgr>`_ to process the data which
+will transform it into a SVG that can be rendered by any web browser.
+
+The following illustrates how to use it:
+
+.. code-block:: console
+
+   ./weston-debug timeline > log.json
+   ./wesgr -i log.json -o log.svg
+
+Inserting timeline points
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Timline points can be inserted using :c:macro:`TL_POINT` macro. The macro will
+take the :type:`weston_compositor` instance, followed by the name of the
+timeline point. What follows next is a variable number of arguments, which
+**must** end with the macro :c:macro:`TLP_END`.
+
+Debug protocol API
+------------------
+
 .. doxygengroup:: debug-protocol
    :content-only: