Update some docs from 2021-10 date triage (rust-lang#1224)

Author: camelid

src/doc/rustc-dev-guide/src/backend/backend-agnostic.md

@@ -2,7 +2,7 @@
 
 <!-- toc -->
 
-As of <!-- date: 2021-01 --> January 2021, `rustc_codegen_ssa` provides an
+As of <!-- date: 2021-10 --> October 2021, `rustc_codegen_ssa` provides an
 abstract interface for all backends to implement, to allow other codegen
 backends (e.g. [Cranelift]).
 

src/doc/rustc-dev-guide/src/contributing.md

@@ -424,12 +424,12 @@ Just a few things to keep in mind:
     Try to format the date as `<MONTH> <YEAR>` to ease search.
 
   - Additionally, include a machine-readable comment of the form `<!-- date:
-    2021-01 -->` (if the current month is January 2021). We have an automated
+    2021-10 -->` (if the current month is October 2021). We have an automated
     tool that uses these (in `ci/date-check`).
 
     So, for the month of January 2021, the comment would look like: `As of <!--
-    date: 2021-01 --> January 2021`. Make sure to put the comment *between* `as of`
-    and `January 2021`; see [PR #1066][rdg#1066] for the rationale.
+    date: 2021-10 --> October 2021`. Make sure to put the comment *between* `as of`
+    and `October 2021`; see [PR #1066][rdg#1066] for the rationale.
 
   - A link to a relevant WG, tracking issue, `rustc` rustdoc page, or similar, that may provide
     further explanation for the change process or a way to verify that the information is not

src/doc/rustc-dev-guide/src/rustdoc-internals.md

@@ -71,29 +71,25 @@ Here is the list of passes as of <!-- date: 2021-02 --> February 2021:
 - `calculate-doc-coverage` calculates information used for the `--show-coverage`
   flag.
 
+- `check-bare-urls` detects links that are not linkified, e.g., in Markdown such as
+  `Go to https://example.com/.` It suggests wrapping the link with angle brackets:
+  `Go to <https://example.com/>.` to linkify it. This is the code behind the
+  <!-- date: 2021-10 --> `rustdoc::bare_urls` lint.
+
 - `check-code-block-syntax` validates syntax inside Rust code blocks
   (<code>```rust</code>)
 
+- `check-doc-test-visibility` runs doctest visibility–related lints.
+
 - `check-invalid-html-tags` detects invalid HTML (like an unclosed `<span>`)
   in doc comments.
 
-- `check-non-autolinks` detects links that could or should be written using
-  angle brackets (the code behind the nightly-only <!-- date: 2021-02 --> `non_autolinks`
-  lint).
-
-- `collapse-docs` concatenates all document attributes into one document
-  attribute. This is necessary because each line of a doc comment is given as a
-  separate doc attribute, and this will combine them into a single string with
-  line breaks between each attribute.
-
 - `collect-intra-doc-links` resolves [intra-doc links](https://doc.rust-lang.org/rustdoc/linking-to-items-by-name.html).
 
 - `collect-trait-impls` collects trait impls for each item in the crate. For
   example, if we define a struct that implements a trait, this pass will note
   that the struct implements that trait.
 
-- `doc-test-lints` runs various lints on the doctests.
-
 - `propagate-doc-cfg` propagates `#[doc(cfg(...))]` to child items.
 
 - `strip-priv-imports` strips all private import statements (`use`, `extern