docs: Add sunos to the reference table

I originally liked "solaris", but I've changed my mind. Both illumos
(the open-source fork of OpenSolaris) and Oracle's closed-source
Solaris are identified by the same token, and there are differences
between them; so using "sunos" as a sort "supertype" for both makes
sense to me.

docs/markdown/Reference-tables.md

@@ -96,6 +96,7 @@ These are provided by the `.system()` method call.
 | linux               | |
 | netbsd              | |
 | windows             | Any version of Windows |
+| sunos               | illumos and Solaris |
 
 Any string not listed above is not guaranteed to remain stable in
 future releases.