Maint: Update docs for schedule type and metaparameter

The wording of these was out of date and somewhat strange.
Maint: Improve/reduce documentation of relationship metaparemeters

These descriptions were too wordy and too inconsistent. I'm imposing a format on
them so they all look alike, and a casual reader can tell how many there are and
how their behaviors differ. I'm also adding links to the more detailed
documentation.
Maint: Standardize docs for refresh behavior in resource types; clarify exec docs

Service/mount/exec already documented that they do the refresh thing, but it
wasn't organized in a predictable way. I'm imposing a convention of a bold
"Refresh" label, a reminder of where events come from, and a description of what
that type does when it gets one.

This commit also adds a lot more detail on how execs respond to refresh, since
it's actually really complicated and not the least bit intuitive. I also reworded
some of the older language near the top of the exec type, while I was in there.
Maint: Clarify docs for stage type and metaparameter

This was pretty disorganized and confusing. Now it's less so!
Merge pull request #2156 from nfagerlund/docs_metaparams_schedule

Docs: Schedule, stage, and other metaparams