Add lots of new code comments.

darcs-hash:20080113164747-75c98-9d0cefd27be7aef7ba60772616d9da7e6bb52912.gz

exec.c

@@ -830,6 +830,9 @@ static pid_t exec_fork()
 }
 
 
+/**
+   Perform output from builtins
+ */
 static void do_builtin_io( wchar_t *out, wchar_t *err )
 {