User's Guide updates

User's Guide updates

Capture various bits of useful information that have come up on the
list but haven't yet gotten into the documentation:

 - Watchdog timers firing during JTAG debug need attention;

 - Some chips have special registers to help JTAG debug;

 - Cortex-M3 stepping example with IRQs and maskisr;

 - Clarifications re adaptive clocking:  not all ARMs do it, and
   explain it a bit better.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>