Document rust-docs-json component

Author: aDotInTheVoid

src/doc/rustdoc/src/unstable-features.md

@@ -465,6 +465,16 @@ Note that the third item is the crate root, which in this case is undocumented.
 [JSON format](https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc_json_types/). `--output-format html` has no effect,
 and is also accepted on stable toolchains.
 
+JSON Output for toolchain crates (`std`, `alloc`, `core`, `test`, and `proc_macro`)
+is available via the `rust-docs-json` rustup component.
+
+```shell
+rustup component add --toolchain nightly rust-docs-json
+```
+
+Then the json files will be present in the `share/doc/rust/json/` directory
+of the rustup toolchain directory.
+
 It can also be used with `--show-coverage`. Take a look at its
 [documentation](#--show-coverage-calculate-the-percentage-of-items-with-documentation) for more
 information.