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

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](https://www.doxygen.nl/manual/commands.html#cmdreturn). 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.

Start by locating clang-doc's JSON backend and its tests for repeated Doxygen special commands. Compare the current output for repeated return comments with the expected single array, then add or update a regression test; done means the comments are emitted as one array while preserving their text order.

Written by the indexing model from the issue text.