Document "invalid channel name '[toolchain]'" error

It took me a while to figure this out. I got this error when building a
project with rust-toolchain containing TOML and I had a fresh rustup,
rust nightly etc. but still there were errors. Running

    rustup default stable
    rustup default nightly

made it work for me. I'm not entirely sure if the explanation is totally
accurate, of course.

Author: jstasiak

doc/src/overrides.md

@@ -1,4 +1,4 @@
-# Overrides
+[[#]] Overrides
 
 `rustup` automatically determines which [toolchain] to use when one of the
 installed commands like `rustc` is executed. There are several ways to control
@@ -97,6 +97,18 @@ just `nightly-2021-01-21`. The file has to be encoded in US-ASCII this case
 (if you are on Windows, check the encoding and that it does not start with a
 BOM). The legacy format is not available in `rust-toolchain.toml` files.
 
+If you ever get an error like the following (when running `rustc`, `cargo` or
+other command)
+
+```
+error: invalid channel name '[toolchain]' in '/PATH/TO/DIRECTORY/rust-toolchain'
+```
+
+it means you're running `rustup` pre-1.23.0 (or just a default toolchain set by
+`rustup` pre-1.23.0) and trying to interact with a project that uses the new TOML
+encoding in the `rust-toolchain` file.  You need to upgrade `rustup` to 1.23.0+
+and set the default toolchain again.
+
 The `rust-toolchain.toml`/`rust-toolchain` files are suitable to check in to
 source control.