This came up at my last Consult gig. In the section for Upgrade config servers to 3.2 CSRS it does not say explicitly that

a) before you upgrade configs to CSRS you need to upgrade mongos to 3.2. This should be step 1 - before stopping the balancer.
b) it would be nice to add a note that MongoDB 's future direction is to de-support old style configDBs - otherwise customers would expect to have a "backout" steps to old config server step. This should be a paragraph after the last step of upgrade - re-enabble balancer

In addition, there are two ways we advice to upgrade to CSRS- one with an outage and one without. These two sections can improve in two ways.

1) Since most of the steps are common, refer "without downtime" to the "with downtime" and mention only the differences to make it happen with no downtime.

2) I like the paragraph for the "without downtime" section clarifying that "no downtime" implies with some limitations --> "During this procedure there will be a period of time where the config servers will be read-only. During this period, certain catalog operations will fail if attempted..." - a similar section should exist for the "with downtime" section too.