talk about --linker

Author: QuietMisdreavus

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

@@ -266,3 +266,16 @@ $ rustdoc src/lib.rs -Z unstable-options --crate-version 1.3.37
 When `rustdoc` receives this flag, it will print an extra "Version (version)" into the sidebar of
 the crate root's docs. You can use this flag to differentiate between different versions of your
 library's documentation.
+
+### `--linker`: control the linker used for documentation tests
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc --test src/lib.rs -Z unstable-options --linker foo
+$ rustdoc --test README.md -Z unstable-options --linker foo
+```
+
+When `rustdoc` runs your documentation tests, it needs to compile and link the tests as executables
+before running them. This flag can be used to change the linker used on these executables. It's
+equivalent to passing `-C linker=foo` to `rustc`.