docs: note about empty environment in child processes

Fixes #1794.

doc/api/child_processes.markdown

@@ -137,6 +137,10 @@ Example of checking for failed exec:
       }
     });
 
+Note that if spawn receives an empty options object, it will result in
+spawning the process with an empty environment rather than using
+`process.env`. This due to backwards compatibility issues with a deprecated
+API.
 
 See also: `child_process.exec()`