The documentation on stringmaps is stored within adt_trie.inc.
Syntax-wise, stringmaps and cookies are pretty much alike, the main difference is that stringmap uses auth-id of a client in the field where a cookie requires an entity name.
In your instance, i think it can be rewritten somehow like this:
public void OnPluginStart() { g_hClientItems[0] = new StringMap(); g_hClientItems[1] = new StringMap(); g_hClientItems[2] = new StringMap(); g_hClientItems[3] = new StringMap(); ... }