Support documentation for comments above arrow functions

[ben-polinsky]

Our current documentation doesn't fully support transforming comments above arrow functions exported like export const functionName... functions. For example: Base64EncodedString.reviver is an exported arrow function - we do not see the comment in source.

However, we can see in the namespace index page the comment does appear: https://www.itwinjs.org/reference/core-common/entities/base64encodedstring/#base64encodedstringreviver