rustdoc-book: Clean up section on --output-format

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

@@ -330,7 +330,7 @@ the source.
 
 ## `--show-type-layout`: add a section to each type's docs describing its memory layout
 
-* Tracking issue: [#113248](https://github.com/rust-lang/rust/issues/113248)
+ * Tracking issue: [#113248](https://github.com/rust-lang/rust/issues/113248)
 
 Using this flag looks like this:
 
@@ -526,9 +526,10 @@ use `-o -`.
 
 ### json
 
+ * Tracking Issue: [#76578](https://github.com/rust-lang/rust/issues/76578)
+
 `--output-format json` emits documentation in the experimental
-[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 format](https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc_json_types/).
 
 JSON Output for toolchain crates (`std`, `alloc`, `core`, `test`, and `proc_macro`)
 is available via the `rust-docs-json` rustup component.
@@ -546,11 +547,11 @@ information.
 
 ### doctest
 
+ * Tracking issue: [#134529](https://github.com/rust-lang/rust/issues/134529)
+
 `--output-format doctest` emits JSON on stdout which gives you information about doctests in the
 provided crate.
 
-Tracking issue: [#134529](https://github.com/rust-lang/rust/issues/134529)
-
 You can use this option like this:
 
 ```bash
@@ -606,6 +607,11 @@ The generated output (formatted) will look like this:
    * `doctest_code` is the code modified by rustdoc that will be run. If there is a fatal syntax error, this field will not be present.
    * `name` is the name generated by rustdoc which represents this doctest.
 
+### html
+
+`--output-format html` has no effect, as the default output is HTML. This is
+accepted on stable, even though the other options for this flag aren't.
+
 ## `--enable-per-target-ignores`: allow `ignore-foo` style filters for doctests
 
  * Tracking issue: [#64245](https://github.com/rust-lang/rust/issues/64245)