Doc update: mention how ARM's WFI instruction affects

JTAG clocking by gating the core clock, and workarounds.
Most details are with the "halt" command, which is one
of the first places this issue will be noticed.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2718 b42882b7-edfa-0310-969c-e2dbd0fdcd60