Add documentation of extended data counts API

[ksen0]

Description of data counts that are going to be exposed. @michaelbrooks @geosoco

[michaelbrooks]

General comments:

• I wonder if range should be changed to days or something with more obvious meaning.
• The bolded parameter in the example docs was intended to indicate that the parameter is required (although I think we should make this explicit rather than using this convention). I suggest we use bold for required parameters and also add "(required)" at the end of the description.
• For those calls with long descriptions, put the long description in a plain line of text after the ### header and use a short description in the ### header. You can see how it looks rendered here. You can also edit and preview on that page directly, rather than guessing what it will render to.

[ksen0]

@michaelbrooks Just saw your general comment. Agree; will do in next commit.