message: Document the patch-in-config feature
diff: @@ -1156,3 +1156,27 @@ absolute path.
  TMPDIR = { value = "/home/tmp", force = true }
  OPENSSL_DIR = { value = "vendor/openssl", relative = true }
  ```
  +
  +### `[patch]` in .cargo/config.toml
  +* Original Pull Request: [#9204](https://github.com/rust-lang/cargo/pull/9204)
  +
  +The `-Z patch-in-config` flag enables the use of `[patch]` sections in
  +cargo configuration files (`.cargo/config.toml`). The format of such
  +`[patch]` sections is identical to the one used in `Cargo.toml`.
  +
  +Since `.cargo/config.toml` files are not usually checked into source
  +control, you should prefer patching using `Cargo.toml` where possible to
  +ensure that other developers can compile your crate in their own
  +environments. Patching through cargo configuration files is generally
  +only appropriate when the patch section is automatically generated by an
  +external build tool.
  +
  +If a given dependency is patched both in a cargo configuration file and
  +a `Cargo.toml` file, the patch in `Cargo.toml` is used. If multiple
  +configuration files patch the same dependency, standard cargo
  +configuration merging is used, which prefers the value defined closest
  +to the current directory, with `$HOME/.cargo/config.toml` taking the
  +lowest precedence.
  +
  +Relative `path` dependencies in such a `[patch]` section are resolved
  +relative to the configuration file they appear in.
  