rename flags per camelid, clarify workflow

* Rename --merge=none|read-write|write-only to --merge=none|shared|finalize
* --write-info-json=target/doc.parts/my-crate/crate-info.json -> --parts-out-dir=target/doc.parts/my-crate
* --include-info-json=target/doc.parts/my-crate/crate-info.json -> --include-parts-dir=target/doc.parts/my-crate
* Clarify the three modes / workflows in the text of the RFC

Author: EtomicBomb