doc: Group adapter sub-commands

doc: Group adapter sub-commands

The commit 5280eb618a8cab46 fixed all `adapter_khz` and
`adapter_nsrst_*` commands in the doc but missed grouping them.  This
let the commands `adapter speed`, `adapter srst pulse_width`, and
`adapter srst delay` not indexed.

Tell texinfo about adapter sub-commands by grouping them in one.

Change-Id: Ida53c4f5cfe28827320c145c8d501d53e831623c
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: http://openocd.zylin.com/6134
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>