Update thread_control_state::trap_expected comments

Update thread_control_state::trap_expected comments

The comments describing trap_expected are out of date.  It
predates displaced stepping and non-stop mode ("keep other threads
stopped").  It predates stepping over watchpoints with breakpoints
inserted (keep_going_pass_signal).  Says the variable is cleared
in normal_stop, when it isn't.  This fixes it.

gdb/ChangeLog:
2020-06-17  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (thread_control_state) <trap_expected> Update
	comments.