Support full doc inheritance

The spec contains @inheritDoc, which has the purpose for object-oriented inheritance of methods where only @params, @returns etc. are copied, but no other tags.

It would be great if it was possible to copy the entire doc of a symbol without copy/pasting it, in order to reduce duplicates and make maintenance easier.

Example case: I'm currently documenting the structure of the parsed result of an XML file using TypeScript interfaces (which are only for typing, they don't get implemented). Here i'm adding comments and other information from the XML reference document about the file. I then create classes to interact with the parsed XML, which should inherit the same documentation about the purpose of fields / properties than those on the interfaces.

Start with the specification’s @inheritDoc behavior and the parser’s handling of doc tags; the issue names no files or tests. Compare interface and class documentation in the XML example, then determine how full-symbol copying should differ from copying @params and @returns. Done means the behavior is specified and supported without duplicated comments, with coverage for the example.