MONGOID-4946 write up discriminator value documentation (#4842)
MONGOID-4946 write up discriminator value documentation (#4842)

* MONGOID-4946 first draft

* MONGOID-4946 change language

* MONGOID-4946 fix structure

* MONGOID-4946 fix title

* MONGOID-4946 more structure fixes

* MONGOID-4946 combine paragraphs

* MONGOID-4946 fix language
mongodb/mongoid,mongodb/mongoid,mongodb/mongoid