Support xmldoc `<inheritdoc>` and `<include>`

[jwosty]

Pretty self-explanatory: it would be nice if the compiler supported <inheritdoc> and <include> elements in xmldoc contexts. The F# docs explicitly mention that this is not supported (see here), and I didn't find an issue already tracking this.

[T-Gro]

I will also create a copy in fslang-suggestions to follow the process and allow everyone to comment.