Fix/Update misc comments

Fix/Update misc comments

While doing some investigation of mine, i noticed a few typos,
inaccuracies and missing information.

I went ahead and updated/improved those.

gdb/ChangeLog:

2020-01-14  Luis Machado  <luis.machado@linaro.org>

	* inf-ptrace.c (inf_ptrace_target::resume): Update comments.
	* infrun.c (resume_1): Likewise.
	(handle_inferior_event): Remove stale comment.
	* linux-nat.c (linux_nat_target::resume): Update comments.
	(save_stop_reason): Likewise.
	(linux_nat_filter_event): Likewise.
	* linux-nat.h (struct lwp_info) <stop_pc>, <stop_reason>: Likewise.