doc: man-page base command

Provide a description of the base command.

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

doc/usage/base.rst

@@ -0,0 +1,23 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+base command
+============
+
+Synopsis
+--------
+
+::
+
+    base [address]
+
+Description
+-----------
+
+The *base* command sets or displays the address offset used by the memory
+commands *cmp, cp, md, mdc, mm, ms, mw, mwc*.
+
+All other commands ignore the address defined by *base*.
+
+address
+    new base address as hexadecimal number. If no value is provided, the current
+    value is displayed.

doc/usage/index.rst

@@ -11,6 +11,7 @@ Shell commands
 .. toctree::
    :maxdepth: 1
 
+   base
    bootefi
    bootmenu
    button