When specifying the "on" field when using $merge, clarify that fields that use a partial index with a unique constraint aren't allowed

Currently, when we specify the "on" field using a field that is partially indexed with a unique constraint, we throw an error informing the user that an index isn't found.

Error: command failed: {

	"ok" : 0,

	"errmsg" : "Cannot find index to verify that join fields will be unique",

	"code" : 51183,

	"codeName" : "Location51183"

} : aggregate failed :

Our source indicates that a field that is partially uniquely indexed is insufficient as a unique index.

It's not clear in our $merge documentation that a partially indexed field is insufficient for specifying "on" in $merge.