Follow-up from SERVER-37715.

This could be supported for dotted paths where only the leaf is multikey. Note however that if any prefix component is multikey, it can't be supported, because every path component is "unwound" in the index, but $unwind only unwinds arrays at the end of the path (e.g. unwind: 'a.b.c' would not unwind arrays on a or a.b).