trace: document 'trace wipe'

Add documentation for the 'trace wipe' command.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/develop/trace.rst

@@ -163,6 +163,17 @@ you will see the time taken by each function shown against its exit record.
               u-boot-1     [000]     3.116466: funcgraph_entry:        0.063 us   |        memset();
               u-boot-1     [000]     3.116539: funcgraph_exit:         0.143 us   |        }
 
+The `trace wipe` command may be used to clear the trace buffer. It leaves
+tracing in its current enable state. This command is convenient when tracing a
+single command, for example:
+
+.. code-block:: console
+
+   => trace pause; trace wipe
+   => trace resume; dhcp; trace pause
+   => trace stats
+   ...
+
 Flame graph
 -----------