Add a bit about on-chain to motivation

Author: Tyera Eulberg

proposals/0106-paritioned-epoch-rewards-pda.md

@@ -30,14 +30,20 @@ rewards were partitioned; in fact, it does not even persist how many blocks the
 rewards distribution spans, so there is no way to predict how long it will take
 or how far through the epoch to go in finding a particular address.
 
+If the partition data were stored in an on-chain account, it would be simple and
+low-compute-cost for a client (with a snapshot or running node) or the runtime
+itself to find the rewards for a particular address for any historical epoch
+after partitioned epoch rewards are activated.
+
 ## Alternatives Considered
 
 An alternative to storing partition data in an on-chain account would be to
 record the necessary data in the ledger in some fashion. This could be a
 transaction that gets added to the block, a special RewardType, or metadata that
 gets stored on the node and then duplicated to long-term storage, like Bigtable.
 In fact, it is probably worthwhile to pursue this alternative as well, since it
-will enable finding rewards without access to snapshots or the running chain.
+will enable finding rewards with just the historical ledger and no access to
+snapshots or the running chain.
 
 ## New Terminology