3.6: Addition of hint to aggregation

A "hint" option has been added to the aggregate command which allows users to force index usage for an aggregation pipeline. We should update both the command and shell helper documentation to reflect. It is worth noting that:

1. Any hint provided affects only the initial document source. It will not be applied for subsequent queries performed by $lookup and $graphLookup.
2. Hint behavior is in line with what we have today for the find command.