move --document-hidden-items to unstable opts section and reword

Author: lolbinarycat

src/doc/rustdoc/src/command-line-arguments.md

@@ -102,15 +102,6 @@ mod private { // this item is private and will not be documented
 
 `--document-private-items` causes all items to be documented as if they were `pub`.
 
-## `--document-hidden-items`: Show items that are `#[doc(hidden)]`
-
-By default, `rustdoc` does not document items that are annotated with
-[`#[doc(hidden)]`](write-documentation/the-doc-attribute.html#hidden).
-
-`--document-hidden-items` causes all items to be documented as if they did not have `#[doc(hidden)]`.
-
-items that are hidden and private will only be documented if both `--document-hidden-items` *and* `--document-private-items` are specified.
-
 ## `-L`/`--library-path`: where to look for dependencies
 
 Using this flag looks like this:

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

@@ -258,6 +258,16 @@ themselves marked as unstable. To use any of these options, pass `-Z unstable-op
 the flag in question to Rustdoc on the command-line. To do this from Cargo, you can either use the
 `RUSTDOCFLAGS` environment variable or the `cargo rustdoc` command.
 
+### `--document-hidden-items`: Show items that are `#[doc(hidden)]`
+<span id="document-hidden-items"></span>
+
+By default, `rustdoc` does not document items that are annotated with
+[`#[doc(hidden)]`](write-documentation/the-doc-attribute.html#hidden).
+
+`--document-hidden-items` causes all items to be documented as if they did not have `#[doc(hidden)]`.
+
+Items that are hidden and private will only be documented if both `--document-hidden-items` *and* `--document-private-items` are specified.
+
 ### `--markdown-before-content`: include rendered Markdown before the content
 
  * Tracking issue: [#44027](https://github.com/rust-lang/rust/issues/44027)

src/doc/rustdoc/src/write-documentation/the-doc-attribute.md

@@ -231,7 +231,7 @@ If you want to know more about inlining rules, take a look at the
 <span id="dochidden"></span>
 
 Any item annotated with `#[doc(hidden)]` will not appear in the documentation,
-unless the `--document-hidden-items` flag is used.
+unless the [`--document-hidden-items`](unstable-features.md#document-hidden-items) flag is used.
 Re-exported items where one of its ancestors has
 `#[doc(hidden)]` will be considered hidden.