DDOC fully-qualified anchors for template members of template structs/classes

This is a supplementary fix of [pull 3693](https://github.com/D-Programming-Language/dmd/pull/3693). The previous pull failed to address the case of a templated member of a templated struct.

This completes the fix for [issue 10366](https://issues.dlang.org/show_bug.cgi?id=10366) (hopefully!).