Document the unique requirements for upgrading more
prominently. Unlike in the past (dating back to the 4.x branch point),
you cannot upgrade from any point on the past couple of stable
branches to -current. Due to a bug in clang that existed before
r286035 in stable/9 or r286033 in stable/10, we cannot compile llvm
that's in 11.x or -current. Unfortunately, these revisions are after
9.3R and 10.3R on their respective branches. stable/11 and 11.0R are
not affected.

This also affects the upgrade path to 11 (including 11.0R) from
stable/9 and stable/10 (which would otherwise work, were it not for
this bug).

We also need to amend the 11.0R release notes.

MFC After: 3 days