doc: document true command

Create a man-page for the true command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/true.rst b/doc/usage/true.rst
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+true command
+============
+
+Synopsis
+--------
+
+::
+
+    true
+
+Description
+-----------
+
+The true command sets the return value $? to 0 (true).
+
+Example
+-------
+
+::
+
+    => true; echo $?
+    0
+    =>
+
+Configuration
+-------------
+
+The true command is only available if CONFIG_HUSH_PARSER=y.