Some of the debugString method of match expressions have newline printed, but some don't.

https://github.com/10gen/mongo/blob/1b27b42504f05c5c8502b83c4d489230eb85c048/src/mongo/db/matcher/expression_expr.cpp#L80-L83
https://github.com/10gen/mongo/blob/1b27b42504f05c5c8502b83c4d489230eb85c048/src/mongo/db/matcher/expression_leaf.cpp#L83-L95

This prints unintuitive string if we have an $and expression over a list of them. We should unify them to make sure they print out consistent newlines.