Document how vector registers are exposed to gdb.

See https://github.com/riscv/riscv-openocd/pull/570

Change-Id: Ie7cdef3717e107a9df0b48316cfbc547dea9a7fd
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6776
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>