Enable MDX descriptions for the API reference to be stored inside JSX blocks

[rdw-software]

Inline MDX on its own would be fine, but without control over the CSS there's no way to render the description at full width. (This is surely going to fail in some edge cases, but I'd rather not mess with it any more than necessary right now.)

---

Spacing messed up because the description isn't inside the block yet:

Description outside the Function block (existing approach), with empty Description block:

Moving the description inside the Function block (new and preferred approach):

Requires the following code changes, which have yet to be applied to all functions:

### path.dirname

Returns the directory path of the given `fileSystemPath` (ignoring trailing separators), similar to the Unix [dirname](https://en.wikipedia.org/wiki/Dirname) command.

<Function since="v0.0.1">
<Parameters>

### path.dirname

<Function since="v0.0.1">
<Description>

Returns the directory path of the given `fileSystemPath` (ignoring trailing separators), similar to the Unix [dirname](https://en.wikipedia.org/wiki/Dirname) command.

</Description>
<Parameters>

Note: Spaces are needed according to the Docusaurus site (MDX parser issue). Otherwise children may be empty?