doc/openocd.texi: Document command mode command

doc/openocd.texi: Document command mode command

Document "command mode" command Server Configuration -> Configuration
Stage.  The text is taken from command's help string.

In addition to the help string, this commit explain the words
mismatch, the doc uses "stage" but the command and source code uses
"mode" to describe the same thing.

Change-Id: I1d5fc8c64a3a0b07ea8430bf016bcbd54e52da1c
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6736
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>