Rustdoc: inconsistent treatment of modules and native modules

[bstrie]

For example, the os module page: http://doc.rust-lang.org/doc/std/os.html

The docs for the module os::libc_constants are split out into their own page ( http://doc.rust-lang.org/doc/std/os_libc_constants.html ) while the two native modules os::libc and os::rustrt just have their docs appended to the bottom of the os page. (Note also that the links at the top of the page to the native module sections are broken.)

Native modules should probably be treated as any other module and be given a page of their own.