exception: Document the three ways we listen for errors/fatals/exceptions

exception: Document the three ways we listen for errors/fatals/exceptions

It is now clear to me why most fatals are logged to 'exception'
on PHP 7, instead of 'fatal' (as on HHVM). It is because these
are, as of PHP 7, technically recoverable if caught locally
with 'catch (Throwable)', and as such should no longer be classified
as 'fatal'. I suppose that's fine and something we'll get used to.

The most important distinction to keep is between 'error' and 'fatal/exception'
given the latter is more heavily monitored and alerted on, but
otherwise they are not usually distingished in query, we treat
them equal for the most part.

Bug: T187147
Change-Id: I64bf0b32dd2648cf72297bdc294e315375329a4d