adjust dev-guide for xtask doctest

Author: tlyu

dev-guide/src/tests.md

@@ -3,7 +3,7 @@
 There are several different kinds of tests you can run (these are enforced in CI):
 
 - [`cargo xtask test-all`](#all-tests) --- Runs all tests.
-- [`mdbook test`](#inline-tests) --- Tests the inline Rust code blocks.
+- [`cargo xtask doctest`](#inline-tests) --- Tests the inline Rust code blocks.
 - [`cargo xtask linkcheck`](#linkcheck) --- Validates that Markdown links aren't broken.
 - [`cargo xtask style-check`](#style-checks) --- Validates various style checks.
 - [Code formatting](#code-formatting) --- Checks that all Rust tooling code is formatted.
@@ -22,11 +22,13 @@ We recommend running this as a last step before opening a PR. This runs most of
 ## Inline tests
 
 ```sh
-mdbook test
+cargo xtask doctest
 ```
 
 This command runs all tests that are inline in the Markdown. Internally, this uses [`rustdoc`](https://doc.rust-lang.org/rustdoc/) to run the tests and supports all the same features. Any code block with the `rust` language will be compiled unless it is ignored. See [Examples] for more.
 
+Previous versions of this guide suggested `mdbook test`, but this only works reliably if your default toolchain is nightly.
+
 ## Linkcheck
 
 ```sh

dev-guide/src/tooling/building.md

@@ -17,7 +17,6 @@ First, ensure that you have a recent copy of the nightly Rust compiler installed
 
 ```sh
 rustup toolchain install nightly
-rustup override set nightly
 ```
 
 Now, ensure you have `mdbook` installed, as this is needed to build the Reference: