corporate: Add some basic documentation of the corporate/* models.
corporate: Add some basic documentation of the corporate/* models.

The billing system has been mostly devoid of any documentation. This
commit is meant to add a basic documentation of the data model of this
system.

andersk/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,rht/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,kou/zulip,zulip/zulip,zulip/zulip,kou/zulip,punchagan/zulip