Currently it only displays explanations of options like --in and --copies.
In the future, it should explain preferred content expression evaluation
and other decisions.
The explanations of a few things could be better. In particular,
"standard" will just appear as-is (or as "!standard" if it doesn't
match), rather than explaining why the standard preferred content expression
for the group matches or not.
Currently as implemented, it goes to stdout, and so commands like
git-annex find that have custom output will not display --explain
information. Perhaps that should change, dunno.
Sponsored-by: Dartmouth College's DANDI project
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