Adding some documentation/comment

include/MySQL_PreparedStatement.h

@@ -4,8 +4,38 @@
 #include "proxysql.h"
 #include "cpp.h"
 
+/*
+One of the main challenge in handling prepared statement (PS) is that a single
+PS could be executed on multiple backends, and on each backend it could have a
+different stmt_id.
+For this reason ProxySQL returns to the client a stmt_id generated by the proxy
+itself, and internally maps client's stmt_id with the backend stmt_id.
 
-// class MySQL_STMTs_local assiciates a global statement ID with a local statement ID for a specific connection
+The implementation in ProxySQL is, simplified, the follow:
+* when a client sends a MYSQL_COM_STMT_PREPARE, ProxySQL executes it to one of
+  the backend
+* the backend returns a stmt_id. This stmt_id is NOT returned to the client. The
+  stmt_id returned from the backend is stored in MySQL_STMTs_local(), and
+  MySQL_STMTs_local() is responsible for mapping the connection's MYSQL_STMT
+  and a global_stmt_id
+* the global_stmt_id is the stmt_id returned to the client
+* the global_stmt_id is used to locate the relevant MySQL_STMT_Global_info() in
+  MySQL_STMT_Manager()
+* MySQL_STMT_Global_info() stores all metadata associated with a PS
+* MySQL_STMT_Manager() is responsible for storing all MySQL_STMT_Global_info()
+  in global structures accessible and shareble by all threads.
+
+To summarie the most important classes:
+* MySQL_STMT_Global_info() stores all metadata associated with a PS
+* MySQL_STMT_Manager() stores all the MySQL_STMT_Global_info(), indexes using
+  a global_stmt_id that iis the stmt_id generated by ProxySQL and returned to
+  the client
+* MySQL_STMTs_local() associate PS located in a backend connection to a
+  global_stmt_id
+*/
+
+
+// class MySQL_STMTs_local associates a global statement ID with a local statement ID for a specific connection
 class MySQL_STMTs_local {
 	private:
 	unsigned int num_entries;