doc: NetPermissionFlags for tx relay in blocksonly

Detail which permission type enables transaction relay for nodes
connected in blockonly mode

Author: willcl-ark

doc/reduce-memory.md

@@ -18,7 +18,7 @@ The size of some in-memory caches can be reduced. As caches trade off memory usa
 
 - Since `0.14.0`, unused memory allocated to the mempool (default: 300MB) is shared with the UTXO cache, so when trying to reduce memory usage you should limit the mempool, with the `-maxmempool` command line argument.
 
-- To disable most of the mempool functionality there is the `-blocksonly` option. This will reduce the default memory usage to 5MB and make the client opt out of receiving (and thus relaying) transactions, except from whitelisted peers and as part of blocks.
+- To disable most of the mempool functionality there is the `-blocksonly` option. This will reduce the default memory usage to 5MB and make the client opt out of receiving (and thus relaying) transactions, except from peers who have the `relay` permission set (e.g. whitelisted peers), and as part of blocks.
 
   - Do not use this when using the client to broadcast transactions as any transaction sent will stick out like a sore thumb, affecting privacy. When used with the wallet it should be combined with `-walletbroadcast=0` and `-spendzeroconfchange=0`. Another mechanism for broadcasting outgoing transactions (if any) should be used.