[www] API docs: render return values

[stefanprobst]

The API docs currently do not render a function's return value defined with jsdoc @returns. This PR fixes this for the simple case where

• a function defines one return value with @returns {type} description
• or multiple return values with the syntax @returns {(type|type|type)] description.

What is not supported is defining multiple return values as separate @returns statements. This currently does not work because of a bug in gatsby-transformer-documentationjs: the description of each @returns statement (DocumentationJSComponentDescription nodes) has the same id and thus all return statements get the description text of the last @returns statement.
What's also not supported is more complex syntax like @returns {Array<number>}, but I think that's not used in the API docs.

Fixes #8203

Edit: Also fix two small errors in the docs.

[m-allanson]

Nice, thanks @stefanprobst 👍