shared-state-async: improve documentation

packages/shared-state-async/README.adoc

@@ -48,8 +48,10 @@ VoCore2: Develop for OpenWrt on Qt Creator
 https://vonger.cn/?p=14657
 
 === Plugin related notes
-Plugins must be registered into shared state by using the config file. UCI infrastructure is preferred
+Plugins must be registered into shared-state-async by using the config file. UCI infrastructure is preferred
 
+[source,console]
+--------------------------------------------------------------------------------
 mSc="plugin_name"
 
 uci set shared-state.${mSc}=dataType
@@ -58,9 +60,11 @@ uci set shared-state.${mSc}.scope='community'
 uci set shared-state.${mSc}.ttl='1200'
 uci set shared-state.${mSc}.update_interval='120'
 uci commit shared-state
+--------------------------------------------------------------------------------
 
-Publishers must be located at "usr/share/shared-state/publishers"
+Publishers must be located at +/usr/share/shared-state/publishers+ 
 All Publishers will be called at least once using shared-state-async-publish-all
 
-Sync is called automatically by shared state according to upgrade interval parameter
-
+Sync is called automatically by shared-state-async according to "update_interval" parameter
+"ttl" stands for "time to live" and will decrease until 0.
+Data contents will be erased if "ttl" reaches 0.