Some minor documentation changes.

A few updates for things that changed recently, some things that were
wrong/missing, and hopefully some clarifications on some matters.

For reviewing, I recommend ignoring the auto-generated .html/.man files and focus on the .md/.adoc files.

Update cargo

24 commits in 0d1f1bbeabd5b43a7f3ecfa16540af8e76d5efb4..34320d212dca8cd27d06ce93c16c6151f46fcf2e
2018-12-19 14:45:14  0000 to 2019-01-03 19:12:38  0000
- Display environment variables for rustc commands (rust-lang/cargo#6492)
- Fix a very minor race condition in `cargo fix`. (rust-lang/cargo#6515)
- Add a high-level overview of how `fix` works. (rust-lang/cargo#6516)
- Add dependency `registry` to `cargo metadata`. (rust-lang/cargo#6500)
- Fix fingerprint calculation for patched deps. (rust-lang/cargo#6493)
- serialize version directly (rust-lang/cargo#6512)
- use DYLD_FALLBACK_LIBRARY_PATH for dylib_path_envvar on macOS (rust-lang/cargo#6355)
- Fix error message when resolving dependencies (rust-lang/cargo#6510)
- use PathBuf in cargo metadata (rust-lang/cargo#6511)
- Fixed link to testsuite in CONTRIBUTING.md (rust-lang/cargo#6506)
- Update display of contents of Cargo.toml (rust-lang/cargo#6501)
- Update display of contents of Cargo.toml (rust-lang/cargo#6502)
- Fixup cargo install's help message (rust-lang/cargo#6495)
- testsuite: Require failing commands to check output. (rust-lang/cargo#6497)
- Delete unnecessary 'return' (rust-lang/cargo#6496)
- Fix new unused patch warning. (rust-lang/cargo#6494)
- Some minor documentation changes. (rust-lang/cargo#6481)
- Add `links` to `cargo metadata`. (rust-lang/cargo#6480)
- Salvaged semver work (rust-lang/cargo#6476)
- Warn on unused patches. (rust-lang/cargo#6470)
- don't write a an incorrect rustc version to the fingerprint file (rust-lang/cargo#6473)
- Rewrite `login` and registry cleanups. (rust-lang/cargo#6466)
- [issue#6461] Fix cargo commands list (rust-lang/cargo#6462)
- Restrict registry names to same style as package names. (rust-lang/cargo#6469)