doc: services: document sys_trace_user_event

Document availability of sys_trace_user_event, for users who are trying
to add custom trace events to their application.

Signed-off-by: Daniel DeGrasse <[email protected]>

doc/services/tracing/index.rst

@@ -32,6 +32,18 @@ interface between payload/format (Top Layer) and the transport mechanics
 (bottom Layer) is generic and efficient enough to model these. See the
 *I/O taxonomy* section below.
 
+User Trace Events
+*****************
+
+Although the user can extend any of the supported serialization formats
+to enable additional tracing functions (or provide their own backend), zephyr
+also provides one generic user tracing function for convenience purposes,
+as well as to demonstrate how tracing frameworks could be extended.
+
+Users can generate a custom trace event by calling
+:c:func:`sys_tracing_user_event`, which takes an event name as well as two
+arbitrary 4 byte arguments. Tracing backends may truncate the provided event
+name if it is too long for the serialization format they support.
 
 Serialization Formats
 **********************