Soon fully replaced by Activity Feed.
As you make allocations you can simultaneously write a detailed log entry about the changes for others to see. These notes can be seen in the Project's Dashboard under Resourcing Log.
As you start allocating in the grid view a field for adding a comment also activates: you can type a comment in this field regarding the allocations and save that and the resourcing changes by clicking Save Changes.
In the project's actions you can find the Resourcing Log ... button:
This view combines all the log entries made for your project: