doc: man-page base command

Provide a description of the base command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/base.rst b/doc/usage/base.rst
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+.. 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.