SERVER-22382, support for standard MongoDB Connection String URIs was added to mongo --host in MongoDB 3.4.
Unfortunately this change also introduced a regression in support for the older syntax for connecting to replica sets via --host (
SERVER-27289, with fix committed for 3.4.2).
The new support for MongoDB URIs should be mentioned:
It would useful to note the regression as well, since a few users have run into this already.