Add a warning for unbound constraints in the validating loader

Closes #2165
Added a new FAQ entry about unbound version constraints
Merge remote-tracking branch 'stof/doc_unbound'
Adjustments to unbound constraint docs
Merge remote-tracking branch 'origin/master'
Add dev-master example
Merge remote-tracking branch 'stof/validate_unbound_constraints'
Only check in require and for non-platform packages, add flag to make this warning optional, refs #2320