doc: describe exit command

Man-page for exit shell command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/usage/exit.rst

@@ -0,0 +1,40 @@
+exit command
+============
+
+Synopsis
+--------
+
+::
+
+    exit
+
+Description
+-----------
+
+The exit command terminates a script started via the run or source command.
+If scripts are nested, only the innermost script is left.
+
+::
+
+    => setenv inner 'echo entry inner; exit; echo inner done'
+    => setenv outer 'echo entry outer; run inner; echo outer done'
+    => run outer
+    entry outer
+    entry inner
+    outer done
+    =>
+
+When executed outside a script a warning is written. Following commands are not
+executed.
+
+::
+
+    => echo first; exit; echo last
+    first
+    exit not allowed from main input shell.
+    =>
+
+Return value
+------------
+
+$? is always set to 0 (true).

doc/usage/index.rst

@@ -17,6 +17,7 @@ Shell commands
    bootefi
    bootmenu
    button
+   exit
    mbr
    pstore
    sbi