Expand write-lock detail

Author: Tyera Eulberg

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

@@ -97,8 +97,14 @@ instead of scanning an unknown number of blocks.
 
 ## Security Considerations
 
-Like traditional sysvars, the new accounts should only be loadable as read-only.
-How this would be accomplished depends on the outcome of SIMD 0105.
+Like traditional sysvars, the partitioned-rewards data PDAs should only be
+loadable as read-only. Read-write loads cannot interfere with instantiation of
+these accounts, since the runtime writes to sysvars before any write locks can
+be taken; however, they could make it difficult for users to read from the
+account on-chain. SIMD 0105 defines a method for demoting sysvar write locks,
+but depends on a list of static addresses. If write-lock handling of dynamically
+addressed sysvars like these PDAs seems needed in the future, a new proposal
+should be introduced.
 
 ## Backwards Compatibility