…-info.json

fixed broken buck2 link, address notriddle's comment about trait vs type
impls, adds period at ends of sentence that are missing periods, and
adds clarification about the usefulness of extern-html-root-url

* 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

I moved the section discussing --extern-html-root-url from unresolved
questions to the Reference-level explanation, as there have been no
proposals discussing alternatives.

I removed the section about no_emit_shared in unresolved questions,
because it has not come up in discussion and no_emit_shared was never a
command line option anyway. The names proposed in discussion are better
and more declarative suggestions

* Explain default behavior of rustdoc in Reference-level explanation
* Flag summary in Guide-level explanation
* Say that crate-info.json is stable because build systems need to rely
  on it
* t,s,i -> trait-crate, struct-crate, index-crate
* T, S -> Trait, Struct

* 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

* New directory: `doc.parts/<crate-name>/crate-info` -> New directory: `doc.parts`.
* Number of times `--parts-out-dir`, `--merge`, and `--include-parts-dir` can be provided are mentioned.
* Describe why `doc.parts` is a directory.
* Description of multiple parallel invocations of rustdoc.
* Describe why to run rustc before rustdoc.
* Fix typos.

* remove 'Unresolved: index crate?' from the unresolved questions section
* remove the requirement on the index crate from various sections

typo in --merge=finalize description: can links

* full sentences in intro
* Clarify Cargo's expected use of `--merge=none|shared|finalize`
* Explain why you would use multiple rustdoc invocations with the same --out-dir
  and merge=none
* Remove justification for why rustdoc output can be unstable in general
* Permalink specific fuchsia commit

Change sentence reading "The WIP may change the sorting order..." to "The implementation may change the sorting order..."

Co-authored-by: Noah Lev <camelidcamel@gmail.com>