Documentation Request Summary:

This should probably be handled as part of documenting the local snapshot reads project in general. However, the docs need to somehow indicate that aggregate can be issued with readConcern level "snapshot" (and that aggregate commands can participate in a mutli-document transaction).

Engineering Ticket Description:

1. aggregate
   1. Including support for $lookup
   2. The $out stage will be prohibited for use within pipelines executed with readConcern level snapshot due to use of metadata operations.