Explain cvar prefixes, rename name_maxlength

Sometimes people ask what the prefixes mean, most are self explanatory but especially the difference between sv_ and g_ is a bit fuzzy and i was never sure what g_ stood for. Is gameplay correct? Any better explanations?

Also renamed a new cvar that was left without a prefix - i think it's better to use them so it's easier to skip uninteresting ones when using apropos. This one is relatively new so it's unlikely any servers are using it.