Clearer docs for path

Author: Jon Gjengset

doc/src/overrides.md

@@ -83,9 +83,6 @@ a directory, the latter is used for backwards compatibility. The files use the
 ``` toml
 [toolchain]
 channel = "nightly-2020-07-10"
-# or
-path = "/path/to/local/toolchain"
-
 components = [ "rustfmt", "rustc-dev" ]
 targets = [ "wasm32-unknown-unknown", "thumbv2-none-eabi" ]
 profile = "minimal"
@@ -107,9 +104,19 @@ The toolchains named in these files have a more restricted form than `rustup`
 toolchains generally, and may only contain the names of the three release
 channels, 'stable', 'beta', 'nightly', Rust version numbers, like '1.0.0', and
 optionally an archive date, like 'nightly-2017-01-01'. They may not name
-custom toolchains, nor host-specific toolchains, except by giving the
-`path` to said toolchain directly. A relative `path` is resolved
-relative to the location of the `rust-toolchain.toml` file.
+custom toolchains, nor host-specific toolchains. To use a custom local
+toolchain, you can instead use a `path` toolchain:
+
+``` toml
+[toolchain]
+path = "/path/to/local/toolchain"
+```
+
+Since a `path` directive directly names a local toolchain, other options
+like `components`, `targets`, and `profile` have no effect. `channel`
+and `path` are mutually exclusive, since a `path` already points to a
+specific toolchain. A relative `path` is resolved relative to the
+location of the `rust-toolchain.toml` file.
 
 ## Default toolchain