Add doxygen for target_resume() ... referencing the still-unresolved
confusion about what the "debug_execution" parameter means (not all
CPU support code acts the same).

The 'handle_breakpoints" param seems to have resolved the main issue
with its semantics, but it wasn't part of the function spec before.

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