Document foreign keys use of idAttribute and not static _id
Document foreign keys use of idAttribute and not static _id

Closes #565

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tgriesser/bookshelf,jordansexton/bookshelf,tgriesser/bookshelf,offlinehacker/bookshelf,chrisbroome/bookshelf,TNuzzi/bookshelf,offlinehacker/bookshelf,rbong/bookshelf,dougshamoo/bookshelf,princesust/bookshelf,princesust/bookshelf,MadcapJake/bookshelf,MadcapJake/bookshelf,lanetix/bookshelf,princesust/bookshelf,TNuzzi/bookshelf,bsiddiqui/bookshelf,stridentbean/bookshelf,patbenatar/bookshelf,chuym/bookshelf,vellotis/bookshelf,shadedprofit/bookshelf,sxhao/bookshelf,joshturn/bookshelf,VideoAmp/bookshelf,shadedprofit/bookshelf,bookshelf/bookshelf,rbong/bookshelf,chrisbroome/bookshelf,offlinehacker/bookshelf,TinOo512/bookshelf,sanchesic89/bookshelf,seegno-forks/bookshelf,chuym/bookshelf,jadengore/bookshelf,tjwudi/bookshelf,nodeonly/bookshelf,apla/bookshelf,jadengore/bookshelf,zubeio/bookshelf,mdboop/bookshelf,TNuzzi/bookshelf,shaunstanislaus/bookshelf,lanetix/bookshelf,bsiddiqui/bookshelf,bookshelf/bookshelf,zubeio/bookshelf,vellotis/bookshelf,sanchesic89/bookshelf,joshturn/bookshelf,skevy/bookshelf,zdespolit/bookshelf,chuym/bookshelf,vellotis/bookshelf,apla/bookshelf,dougshamoo/bookshelf,shaunstanislaus/bookshelf,sanchesic89/bookshelf,jadengore/bookshelf,zdespolit/bookshelf,tjwudi/bookshelf,cspotcode/bookshelf,stonegithubs/bookshelf,patbenatar/bookshelf,skevy/bookshelf,skevy/bookshelf,TinOo512/bookshelf,stonegithubs/bookshelf,joshturn/bookshelf,jordansexton/bookshelf,lanetix/bookshelf,tjwudi/bookshelf,mdboop/bookshelf,tbutman/bookshelf,tbutman/bookshelf,MadcapJake/bookshelf,stridentbean/bookshelf,zubeio/bookshelf,VideoAmp/bookshelf,nodeonly/bookshelf,sxhao/bookshelf,patbenatar/bookshelf,dougshamoo/bookshelf,sxhao/bookshelf,shadedprofit/bookshelf,stonegithubs/bookshelf,shaunstanislaus/bookshelf,zdespolit/bookshelf,seegno-forks/bookshelf,mdboop/bookshelf,nodeonly/bookshelf,stridentbean/bookshelf,tbutman/bookshelf,VideoAmp/bookshelf,cspotcode/bookshelf