We now have an \externalpage pointing to https://en.cppreference.com/w/cpp/symbol_index/chrono_literals, use it to consistently link to it instead of having each documentation user of 1s or 50ns explain the concept anew.

This does not overlap with QTBUG-138535, because that would automatically link std::chrono::duration, but not "C++14's literals for time", so it can be done in parallel, without waiting for QTBUG-138535.