Document that linkcheck only looks at modified files when running locally

when i run
RUST_LOG=trace mdbook build
I get:

2021-01-22 08:35:53 [WARN] (mdbook::renderer): Renderer command `../../ci/linkcheck.sh`
uses a path relative to the renderer output directory `/Users/azw/repos/rustc-dev-guide/book/linkcheck`. This was previously accepted, but has been deprecated. Relative executable paths should be relative to the book root.
Checking files changed in master...: src/about-this-guide.md src/contributing.md src/git.md
[2021-01-22T16:35:53Z DEBUG mdbook_linkcheck] Loading cache from /Users/azw/repos/rustc-dev-guide/book/linkcheck/cache.json
[2021-01-22T16:35:53Z INFO  mdbook_linkcheck] Started the link checker
[2021-01-22T16:35:53Z DEBUG mdbook_linkcheck] Selected file: Some(["src/about-this-guide.md", "src/contributing.md", "src/git.md"])
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck] Config {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     follow_web_links: true,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     traverse_parent_directories: false,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     exclude: [
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "crates\\.io",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: crates\.io,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "gcc\\.godbolt\\.org",[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: gcc\.godbolt\.org,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "youtube\\.com",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: youtube\.com,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "youtu\\.be",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: youtu\.be,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "dl\\.acm\\.org",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: dl\.acm\.org,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "cs\\.bgu\\.ac\\.il",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: cs\.bgu\.ac\.il,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "www\\.amazon\\.com",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: www\.amazon\.com,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "www\\.rustaceans\\.org",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: www\.rustaceans\.org,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         HashedRegex {
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             string: "play\\.rust-lang\\.org",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]             re: play\.rust-lang\.org,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]         },
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     ],
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     user_agent: "mdbook-linkcheck-0.7.2",
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     cache_timeout: 86400,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     warning_policy: Error,
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck]     http_headers: {},
[2021-01-22T16:35:53Z TRACE mdbook_linkcheck] }
[2021-01-22T16:35:53Z INFO  mdbook_linkcheck] Scanning book for links
[2021-01-22T16:35:53Z INFO  mdbook_linkcheck] Found 0 links (0 incomplete links)
[2021-01-22T16:35:53Z TRACE mio::sys::unix::kqueue] registering; token=Token(18446744073709551615); interests=Readable
[2021-01-22T16:35:53Z TRACE mio::poll] registering with poller
[2021-01-22T16:35:53Z DEBUG mdbook_linkcheck] Saving the cache to /Users/azw/repos/rustc-dev-guide/book/linkcheck/cache.json
[2021-01-22T16:35:53Z INFO  mdbook_linkcheck] No broken links found
2021-01-22 08:35:53 [TRACE] (mdbook::renderer): ../../ci/linkcheck.sh exited with output: ExitStatus(ExitStatus(0))

which says it found no links in the book at all. Separately, when i manually run
RUST_LOG=trace mdbook-linkcheck -f src/git.md book or RUST_LOG=trace ../../ci/linkcheck.s it just hangs? Have I set up my local instance incorrectly? I think its very curious that it seems to be running the same thing

if I remove the command = from the linkcheck section of the book.toml, linkcheck works, but I get the classic Server returned 429 Too Many Requests for for github as well as some other interesting errors

Start with the linkcheck section in book.toml and the local commands shown in the issue. Confirm why local linkcheck selects modified files and how the manual commands behave, then document the expected behavior and setup, including what a successful local run should report.