Improve the explanation of accounts (buyers/merchants)

[matin]

The current explanation of the accounts resource in the docs and the api spec is pretty atrocious. Specifically the concept of underwriting a merchant is pretty foreign to most payments services.

Balanced underwrites sellers/recipients as merchants to comply with payment aggregation compliance policies. Otherwise, the marketplace would become an aggregator. There's a explanation of the ramifications on Gittip #67.

The API has to validate the identity of the seller to underwrite them as a merchant, which requires, at minimum, the legal name, home address, and DOB. This information is verified against external data sources, but Balanced doesn't currently explain this process well or return very useful messages in the API response.

[jrus]

Migrating this to balanced/balanced-docs#6