Add Base.Sys.which to docs (#57835)

Looks like this is probably missing by accident. It is public already.

Author: frankier

doc/src/base/base.md

@@ -397,6 +397,7 @@ Base.Sys.loadavg
 Base.Sys.isexecutable
 Base.Sys.isreadable
 Base.Sys.iswritable
+Base.Sys.which
 Base.Sys.username
 Base.@static
 ```