Tweak formatting and style of _find API documentation (#4460)

While reading the documentation, noticed a couple of problems with
the HTML version, of which this change covers:

- Parameter types for JSON objects are not uniform.  The type `json`
  could either ambigiously refer to objects or arrays and used
  randomly.
- The `Content-Type` header for response descriptions is not always
  put in a list, though probably that is the right choice because
  there can be many.
- Indentation of paragraphs and list items is off sometimes which
  causes rendering mistakes.
- Formatting of JSON examples is not uniform by indentation.
- Add a couple of missed markdown for names, such as operators,
  to make them stand out from the text for better readability.
- Unify style: use colons for the sentences that introduce
  examples, end sentences with full stops, lose contractions,
  regularize elaboration of object details for index descriptions
  and definitions.
- Fix the reference to views in design documents when talking
  about indexes.
- Add link for `_find` at the section about the index selection.