doc: add comments about unit of measure of partition size

air/src/options.rs

@@ -377,6 +377,7 @@ impl PartitionOptions {
 
     /// Returns the size of each partition used when committing to the main and auxiliary traces as
     /// well as the constraint evaluation trace.
+    /// The returned size is given in terms of number of columns in the field `E`.
     pub fn partition_size<E: FieldElement>(&self, num_columns: usize) -> usize {
         if self.num_partitions == 1 && self.min_partition_size == 1 {
             return num_columns;