trace doc: convert trace/stm.txt to rst format

This converts the plain text documentation to reStructuredText format and
add it into Sphinx TOC tree. No essential content change.

Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/index.rst

@@ -20,3 +20,4 @@ Linux Tracing Technologies
    mmiotrace
    hwlat_detector
    intel_th
+   stm

Documentation/trace/stm.rst

@@ -1,3 +1,4 @@
+===================
 System Trace Module
 ===================
 
@@ -32,7 +33,7 @@ associated with it, located in "stp-policy" subsystem directory in
 configfs. The topmost directory's name (the policy) is formatted as
 the STM device name to which this policy applies and and arbitrary
 string identifier separated by a stop. From the examle above, a rule
-may look like this:
+may look like this::
 
 	$ ls /config/stp-policy/dummy_stm.my-policy/user
 	channels masters
@@ -78,7 +79,7 @@ stm_source
 For kernel-based trace sources, there is "stm_source" device
 class. Devices of this class can be connected and disconnected to/from
 stm devices at runtime via a sysfs attribute called "stm_source_link"
-by writing the name of the desired stm device there, for example:
+by writing the name of the desired stm device there, for example::
 
 	$ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link
 
@@ -118,5 +119,5 @@ the same time.
 
 Currently only Ftrace "function" tracer is supported.
 
-[1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
-[2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html
+* [1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
+* [2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html