Document how to skip suite-state polling about cylc-doc

[arjclark]

@hjoliver - this one's so hard to know what to do with as its so dependent on what the desired behaviour would be. I think often, as you say, the polling task is used as a blocker so that later tasks that assume the presence of a particular file don't get run. Quite like the idea of expiry but only in the context of "if I'm late I might be ok to run a file check instead before proceeding with my workflow".

The only code based enhancement I can think of would be to finesse the poller in some way so you could provide an option to treat polling for cycle points that would be before the cold/warm point of a recently restarted as "successful" in a similar (but definitely not equivalent as its never treated as a task success) manner to how dependencies on pre-start points get stripped out - perhaps a "--succeed-if-prestart" option?

[hjoliver]

+1 for @arjclark's last suggestion above, from a user at NIWA. (Although I'm not sure of the impact: would we have to look up the earliest cycle point every time a poll fails, and is that easy and quick to do?)

[arjclark]

@hjoliver - easy: yes, quick: not necessarily. I can think of several ways to do it and you could potentially make things easier by providing a cutoff -i.e. --succeed-pre YYYYMMDDTHHZ type option on the invocation side for when you're using either a generic script of the graphing inlined poller task if you know that the suite won't have any entries prior to that.