Make solarispkg work with multi-pkg for pkg.latest

Currently solarispkg doesn't support repositories for pkgadd, so these
changes won't really allow pkg.latest to work as intended. This commit
does however bring this module in line with the changes being made in
other providers for #2906.
Update docstring for pkg.latest

This update reflects the providers that have had the proper changes made
to them to allow multi-pkg support in pkg.latest.
Merge pull request #3208 from archtaku/issue2906

Make solarispkg work with multi-pkg for pkg.latest