rustdoc: add docs for --emit

Author: notriddle

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

@@ -269,6 +269,19 @@ Markdown file, the URL given to `--markdown-playground-url` will take precedence
 `--playground-url` and `#![doc(html_playground_url = "url")]` are present when rendering crate docs,
 the attribute will take precedence.
 
+### `--emit`: Control rustdoc output
+
+Assigned to a comma-separated list of formats to output:
+
+- `invocation-specific`: Write files that are specific to the current crate (to `--out-dir`.
+- `toolchain-shared-resources`: Write output that is the same for any crate to `--out-dir`. This only applies to HTML, which relies on static files for styling and functionality.
+- `dep-info[=FILENAME]`: Write a file (in Makefile-compatible syntax) listing every file or environment variable that affects the output.
+
+By default, Rustdoc emits `invocation-specific,toolchain-shared-resources`.
+When this parameter is supplied, it overrides the default, so
+`--emit=invocation-specific,toolchain-specific,dep-info` writes everything, and
+`--emit=dep-info` only writes dep-info.
+
 ## `--sort-modules-by-appearance`: control how items on module pages are sorted
 
 Using this flag looks like this: