Correct some comments.

sys/arch/pmax/conf/std.pmax

@@ -2,15 +2,12 @@
 # standard, required pmax info
 
 machine pmax mips
-mainbus0 at root
-cpu* at mainbus0
 
 # Non-optional options
 options 	SWAPPAGER	# paging
 options 	VNODEPAGER	# mmap() of files
 options 	DEVPAGER	# mmap() of devices
 
-
 # Standard exec-package options
-options 	EXEC_ELF32	# native exec format
-options 	EXEC_SCRIPT	# may be unsafe
+options 	EXEC_ELF32	# 32-bit ELF support (native format)
+options 	EXEC_SCRIPT	# exec of #! scripts