[clang-doc] Certain comment types should be single paragraphs

[evelez7]

Certain Doxygen special commands are concatenated regardless of the order they're written. For example:

/// \return Foo
/// \brief Brief!
/// \return Bar

will look like:

*Return*
Foo
Bar

See the return special command. Currently, the JSON backend treats these as separate comment paragraphs, and the JSON looks like the following:

"ReturnComments": [
  [
    "TextComment": "Foo"
  ],
  [
    "TextComment": "Bar
  ]
]

This JSON should be emitted as a single array.