Problems referring to QT_INSTALL_DOCS in .qdocconf files

Any reference to $QT_INSTALL_DOCS in .qdocconf files will point to the source directory. This is intentional, making it possible to always include configuration files under doc/global (see 23d8a7fb).

However, in some modules outside Qt, we'd need a possibility to also load index files for Qt module docs, and those are always built in the actual build directory for docs.

So this is more a feature request, I guess - could we get an env variable for qdoc to refer to the actual QT_INSTALL_DOCS?