Put appropriate "(Why?)" links in the user guide as we did for the javadoc

Please spent a short paragraph in Nullness User Guide why I have to write
<T extends @Nullable Object> T firstOrDefault(List<T> list, T defaultValue) {
instead of
<@Nullable T> T firstOrDefault(List<T> list, T defaultValue) {
and / or provide a comprehensible explanation what latter actually means.

(Out of scope: is it possible to pass Consumer<@Nullable String> c to void process(Consumer<String> s))

Start with the Nullness User Guide and inspect how the javadoc uses “(Why?)” links. Add a short, comprehensible explanation of why the two generic nullness forms differ, or what the latter means, while leaving the Consumer question out of scope; the issue is done when the relevant guide section contains that explanation and/or link.