Help: Document site_name command use of HOSTNAME variable

Help/command/site_name.rst

@@ -6,3 +6,7 @@ Set the given variable to the name of the computer.
 .. code-block:: cmake
 
   site_name(variable)
+
+On UNIX-like platforms, if the variable ``HOSTNAME`` is set, its value
+will be executed as a command expected to print out the host name,
+much like the ``hostname`` command-line tool.