MONGOID-4934 discriminator key documentation (#4830)
MONGOID-4934 discriminator key documentation (#4830)

* MONGOID-4934 discriminator key docs first draft

* MONGOID-4934 fixes

* MONGOID-4934 removed header, changed to subheader

* MONGOID-4934 typos and rewording

* MONGOID-4934 fix numbered list

* MONGOID-4934 numbered lists

* MONGOID-4934 typos and rewording

* MONGOID-4934 second draft

* MONGOID-4934 small language update

* MONGOID-4934 fix up language
mongodb/mongoid,mongodb/mongoid,mongodb/mongoid