mod_blocklist: Add comment to clarify some logic

plugins/mod_blocklist.lua

@@ -189,6 +189,7 @@ local function edit_blocklist(event)
 
 	if is_blocking then
 		for jid in pairs(send_unavailable) do
+			-- Check that this JID isn't already blocked, i.e. this is not a change
 			if not blocklist[jid] then
 				for _, session in pairs(sessions[username].sessions) do
 					if session.presence then