rustdoc: link to stable/beta docs consistently in documentation

## User-facing changes

- Intra-doc links to primitives that currently go to rust-lang.org/nightly/std/primitive.x.html will start going to channel that rustdoc was built with. Nightly will continue going to /nightly; Beta will link to /beta; stable compilers will link to /1.52.1 (or whatever version they were built as).
- Cross-crate links from std to core currently go to /nightly unconditionally. They will start going to /1.52.0 on stable channels (but remain the same on nightly channels).
- Intra-crate links from std to std (or core to core) currently go to the same URL they are hosted at; they will continue to do so. Notably, this is different from everything else because it can preserve the distinction between /stable and /1.52.0 by using relative links.

Note that "links" includes both intra-doc links and rustdoc's own
automatically generated hyperlinks.

 ## Implementation changes

- Update the testsuite to allow linking to /beta and /1.52.1 in docs
- Use an html_root_url for the standard library that's dependent on the channel

  This avoids linking to nightly docs on stable.

- Update rustdoc to use channel-dependent links for primitives from an
  unknown crate

- Set DOC_RUST_LANG_ORG_CHANNEL from bootstrap to ensure it's in sync
- Include doc.rust-lang.org in the channel

library/term/src/lib.rs

@@ -30,11 +30,7 @@
 //! [win]: https://docs.microsoft.com/en-us/windows/console/character-mode-applications
 //! [ti]: https://en.wikipedia.org/wiki/Terminfo
 
-#![doc(
-    html_root_url = "https://doc.rust-lang.org/nightly/",
-    html_playground_url = "https://play.rust-lang.org/",
-    test(attr(deny(warnings)))
-)]
+#![doc(html_playground_url = "https://play.rust-lang.org/", test(attr(deny(warnings))))]
 #![deny(missing_docs)]
 #![cfg_attr(windows, feature(libc))]