event: Add documentation

Add documentation for events, including the event command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/event.rst b/doc/usage/event.rst
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+.. SPDX-License-Identifier: GPL-2.0+
+
+event command
+=============
+
+Synopsis
+--------
+
+::
+
+    event list
+
+Description
+-----------
+
+The event command provides spy list.
+
+This shows the following information:
+
+Seq
+    Sequence number of the spy, numbered from 0
+
+Type
+    Type of the spy, both as a number and a label. If `CONFIG_EVENT_DEBUG` is
+    not enabled, the label just shows `(unknown)`.
+
+Function
+    Address of the function to call
+
+ID
+    ID string for this event, if `CONFIG_EVENT_DEBUG` is enabled. Otherwise this
+    just shows `?`.
+
+
+See :doc:`../develop/event` for more information on events.
+
+Example
+-------
+
+::
+
+    => event list
+    Seq  Type                              Function  ID
+      0  7   misc_init_f               55a070517c68  ?
+
+Configuration
+-------------
+
+The event command is only available if CONFIG_CMD_EVENT=y.