Per this comment on SERVER-38174, we should update the documentation around maintenance of replica sets to clarify a few points.

1. Beginning in MongoDB 4.0, when restarting members of replica sets in standalone mode (without the --replset argument),
   1. The node will not provide read access to the most current data it possesses, but only to data that is no more recent that the majority commit point, unless
   2. the node is started with the recoverFromOplogAsStandalone flag, in which case the node will present all data it has accepted, but will not accept write operations.
2. In all versions of MongoDB,
   1. performing write operations on replica set members running in standalone mode risks data corruption
   2. and only the following operations are endorsed, though even they may lead to corruption if not used carefully:
      1. Building indexes, with caveats around the danger of building unique indexes in this manner
      2. Manipulating the contents of the local database, with caveats around the risks of updating collections involved in the replication process

Further, we should review any documentation we have that involves doing maintenance on replica set members in standalone mode to ensure they are correct, or can be safely performed.