The returnKey and maxScan options to find are passed using a different format for legacy OP_QUERY find and the find command (new in 3.2). Therefore, passing a literal $returnKey or $maxScan as part of the argument to a .find() is no longer supported in the shell:

./mongo --readMode commands
> db.c.drop()
> db.c.insert({})
> db.c.find({$query: {}, $maxScan: 5})
assert: command failed: {
	"waitedMS" : NumberLong(0),
	"ok" : 0,
	"errmsg" : "unknown top level operator: $query",
	"code" : 2
}

Commit ffdb57ed20b introduced cursor.returnKey() and cursor.maxScan() helpers, which are now the official way to set these options. In the 3.2 manual, we should replace the $returnKey documentation (http://docs.mongodb.org/manual/reference/operator/meta/returnKey/) with a page documenting the cursor.returnKey() method. Similarly, we should replace http://docs.mongodb.org/manual/reference/operator/meta/maxScan/ with a page documenting the cursor.maxScan() method.