jtag: replace command "jtag_reset" with "adapter [de]assert"

jtag: replace command "jtag_reset" with "adapter [de]assert"

Replace the JTAG transport specific command with a more generic
one. Deprecate "jtag_reset" and update the documentation.
While there, fix an error in the documentation, where the command
"jtag_reset" was used in place of command "reset_config".

Change-Id: I41a988d37ce69f7b35a960cbaf5306aab0299b99
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5286
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>