State documentation uses outdated example for user discriminator query

[jchristgit]

As brought up by @jb3, the state documentation currently mentions an example of
querying users within nosedrum by matching via username and discriminator,
whilst discriminator for active user accounts are no longer in use. This might
have to be fixed in nosedrum itself, but in the meantime we could update the
documentation to use the username attribute only as it is supposed to be unique
for user accounts these days. A question for @jb3 here, could the query return
multiple users then, if you query for, for instance, an inactive user?

[jb3]

I'm not sure what you mean by an inactive user here, but if you can clarify I can try answer.

I suggest we just swap out the query for something else that demonstrates more advanced state/QLC usage for cache querying, querying by just username does not feel like a great example, let's at least look for more fields.

I think we can show some more example uses, just a few off the top of my head (though I have tested them, and they do work), let me know what you think of these (they may also be a totally wrong approach to writing QLC queries, let me know if that is the case also):

-module(nostrum_queries).
-export([find_role_users/4, find_large_communities/2]).

-include_lib("stdlib/include/qlc.hrl").

% Find the Nostrum.Struct.User and Member objects of all members in a specific guild role.
find_role_users(RequestedGuildId, RoleId, MemberCache, UserCache) ->
    qlc:q([{User, Member} || {{GuildId, MemberId}, Member} <- MemberCache:query_handle(),
                   % Filter to member objects of the selected guild
                   GuildId =:= RequestedGuildId,
                   % Filter to members of the provided role
                   lists:member(RoleId, map_get(roles, Member)),
                   % Get a handle on the UserCache table
                   {UserId, User} <- UserCache:query_handle(),
                   % Find the User struct that matches the found Member
                   MemberId =:= UserId]).

% Find all communities in the Guild cache with the COMMUNITY guild feature
% that are over a certain threshold in user size
find_large_communities(Threshold, GuildCache) ->
    qlc:q([Guild || {_, Guild} <- GuildCache:query_handle(),
                    % Filter for guilds that are over the provided size
                    map_get(member_count, Guild) > Threshold,
                    % Filter for guilds that have COMMUNITY in the features field
                    lists:member(<<"COMMUNITY">>, map_get(features, Guild))]).

(Related, QLC cheatsheet for #553?)

[jchristgit]

I like this idea and also the idea of a QLC cheatsheet, as far as I'm aware the queries above won't work as-is since the QLC compiler expects guard tests to be passed in the list comprehension, so a lists:member call does not work as far as I'm aware

[jb3]

Discussion in Discord shows that we can use lists:member and it does seem to be doing this efficiently. I think we're good to put in those two examples into the state documentation as well as a QLC cheatsheet (I can add into #559).