dumpstate: notes on usage

This is what a few old maintainers of dumpstate told me
when I made changes to it back ~2016 or so: always exec
stuff, never link it.

Well, there have been quite a few divergences from this,
and it's come up in a couple of reviews I've seen here.

So, I've added notes on this into the readme and comments
on some of the worst offender. For instance, we could
have probably avoided a lot of this async/exec logic
for these custom cases if we used the standard run
command with timeout stuff.

Bug: N/A
Test: N/A
Change-Id: I4c33f3042ca16d4433082b8d78ebdd29cb75f0b3