Clarify "store data", and more generally make normative statements self-explanatory (as much as possible)

Replace words in Section 4.1 (Storing state) Prior to non-normative example. It is necessary for the Client and Server to store Session state in order to provide Quality of Service guarantees. The Client and Server MUST store Session state for the duration of the Session [MQTT-xyz] . A Session MUST last at least as long it has an active Network Connection [MQTT-xyz] . Retained messages do not form part of the Session state in the Server. The Server SHOULD retain such messages until deleted by a Client. Non normative comment. The storage capabilities of Client and Server implementations will of course have limits in terms of capacity and may be subject to administrative policies such as the maximum time that Session state is stored between Network Connections. Stored Session state can be discarded as a result of an administrator action, including an automated response to defined conditions. This has the effect of terminating the Session. These actions might be prompted by resource constraints or for other operational reasons. It is prudent to evaluate the storage capabilities of the Client and Server to ensure that they are sufficient. Non normative comment. It is possible that hardware or software failures may result in loss or corruption of Session state stored by the Client or Server. >>> The non-normative example should stand <<<.