Fix doc error with rust-toolchain.toml custom TC

Fix an issue where the documentation erroneously states that
`rust-toolchain.toml` config files can't specify custom toolchain names.

This language seems to be very old, ultimately inherited from the
original commit that added this feature, and is no longer true:
107d8e5#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5R330

doc/user-guide/src/overrides.md

@@ -120,16 +120,12 @@ The `channel` setting specifies which [toolchain] to use. The value is a
 string in the following form:
 
 ```
-<channel>[-<date>]
+(<channel>[-<date>])|<custom toolchain name>
 
 <channel>       = stable|beta|nightly|<major.minor.patch>
 <date>          = YYYY-MM-DD
 ```
 
-Note that this is a more restricted form than `rustup` toolchains
-generally, and cannot be used to specify custom toolchains or
-host-specific toolchains.
-
 [toolchain]: concepts/toolchains.md
 
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