doc: clarify -i2pacceptincoming help documentation

and also hoist the default setting to a constexpr and
remove unused f-string operators in a related functional test.

test/functional/p2p_i2p_sessions.py

@@ -23,12 +23,12 @@ class I2PSessions(BitcoinTestFramework):
 
         self.log.info("Ensure we create a persistent session when -i2pacceptincoming=1")
         node0 = self.nodes[0]
-        with node0.assert_debug_log(expected_msgs=[f"Creating persistent SAM session"]):
+        with node0.assert_debug_log(expected_msgs=["Creating persistent SAM session"]):
             node0.addnode(node=addr, command="onetry")
 
         self.log.info("Ensure we create a transient session when -i2pacceptincoming=0")
         node1 = self.nodes[1]
-        with node1.assert_debug_log(expected_msgs=[f"Creating transient SAM session"]):
+        with node1.assert_debug_log(expected_msgs=["Creating transient SAM session"]):
             node1.addnode(node=addr, command="onetry")