This question is specifically on capability to enter comments directly in a cell. The user case would be budget Vs actual variance report where i want to see commentary for a specific line. I have been told that this isn't possible , as we can only enter comments in the page/grid which is not specifically linked to a cell
This approach would be the simplest if you P&L is a list and not line items.
My suggestion in creating an output where you convert you line items into a line item subset is one way of converting your line items into a list so that you can follow this approach as suggested by @filip.sypniewski .
Either way the challenge is allowing the module to contain both numerical and text formatted data against the same intersection of dimensions.
How about creating a list called Comments with a single list item.
Create a new module and add all relevant dimensions that are present in the variance report along with a line item subset for the source module.
Add a single line item called, 'Comments.'
Publish this to a page using a field card.
The card should be synchronised to the variance report and show the relevant comment when the user selects a line item in the report.
If you need to publish a report with the contents use the line item subset and the dimensions along with a line item for the values and one for the comment. Simply reference each module to combine them in a single output.