Misc xtrigger and ID doc updates

[hjoliver]

User feedback: the workflow_state xtrigger is poorly documented,

• what's workflow_task_id (IMO that is documented, but perhaps we could link to general ID docs)
• what's "selector" (again, link to ID docs) and what's its default, and it should renamed "selection" (disagreed)
• what exactly is the "offset"
• why are the is_selector and is_message booleans needed, can't we just default to status or whatever?

Conclusions, after some follow-up doc trolling:

• the workflow-state command is much better documented than the xtrigger; we should fix that and/or reference command help
• ID should be documented early in the User Guide, not just in the Cylc 8 migration section
• the glossary includes workflow ID, but not task ID!
• cylc help id refers to "filters" not "selector" and mentions states but not outputs.

[hjoliver]

See also #820 - maybe fix the xtrigger bits with that.