[PIE-1617] Added tx-pool-retention-hours CLI option (#1481)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

docs/Reference/Pantheon-CLI-Syntax.md

@@ -1017,6 +1017,22 @@ tx-pool-max-size="2000"
 
 Maximum number of transactions kept in the transaction pool. Default is 4096. 
 
+### tx-pool-retention-hours
+
+```bash tab="Syntax"
+--tx-pool-retention-hours=<INTEGER>
+```
+
+```bash tab="Example Command Line"
+--tx-pool-retention-hours=5
+```
+
+```bash tab="Example Configuration File"
+tx-pool-retention-hours="5"
+```
+
+Maximum period in hours to retain pending transactions in the transaction pool. Default is 13. 
+
 ### help
 
 ```bash tab="Syntax"

docs/Using-Pantheon/Transactions/Transaction-Pool.md

@@ -8,6 +8,9 @@ transactions in the node transaction pool
 * [`--tx-pool-max-size`](../../Reference/Pantheon-CLI-Syntax.md#tx-pool-max-size) command line option to specify the maximum number
 of transactions in the node transaction pool
 
+* [`tx-pool-retention-hours`](../../Reference/Pantheon-CLI-Syntax.md#tx-pool-retention-hours) command line option to specify 
+the maximum number of hours to retain pending transactions in the transaction pool
+
 * [`newPendingTransactions`](../../Pantheon-API/RPC-PubSub.md#pending-transactions) and [`droppedPendingTransactions`](../../Pantheon-API/RPC-PubSub.md#dropped-transactions)
 RPC subscriptions to notify of transactions added to and dropped from the node transaction pool