Add more detail to data standard docs

[Jason Vander Heiden]

It came up in the AIRR formats working group call that the docs for the data standard are not explicit enough. We need to add details about the delimiters, how nesting works, and where the parsing API is, etc.

[Jason Vander Heiden]

Done (enough) in 11dfd78. Mostly through populating the Attributes element of the Receptor.Receptor docstring. This will all have to be updated when AIRR is finalized, but good enough for now.