del --include-rendered-docs, doc.parts suggestions

* Remove the --include-rendered-docs flag
* --parts-out-dir restricted to --merge=none
* --include-parts-dir restricted to --merge=finalize
* crate-info.json -> crate-info to emphasize opaqueness
* doc.parts contents edition stability promises are
  replaced with a statement saying that
  rustdoc will make an reasonable effort to continue
  just having crate-info files

Author: EtomicBomb