For a long time, the Solaris support in chrony wasn't tested on a real
Solaris system, but on illumos/OpenIndiana, which was forked from
OpenSolaris when it was discontinued in 2010.

While Solaris and illumos might have not diverged enough to make a
difference for chrony, replace Solaris in the documentation with illumos
to make it clear which system is actually supported by the chrony
project.