doc: CodeReview.md: Clarify cfg usage

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

chips/lowrisc/Cargo.toml

@@ -11,3 +11,7 @@ edition.workspace = true
 [dependencies]
 rv32i = { path = "../../arch/rv32i" }
 kernel = { path = "../../kernel" }
+
+[lints.rust]
+# These are unused and unsupported
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(aon_wkup_timer)'] }

doc/CodeReview.md

@@ -344,6 +344,12 @@ Generally, this means `cfg` directives should be an explicit list of chips or'd
 together. Rarely, if ever, is a `cfg(not ...)` the correct approach for anything
 outside of unit tests.
 
+Rust `cfg` features can also be used to disable code that isn't used in upstream
+Tock but might be used in private or public forks. Rust custom configs can also
+be used, but generally require changes to the lint configuration. See
+https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html#check-cfg-in-lintsrust-table
+for more details.
+
 #### Boards
 
 Changes to boards are generally left to the maintainer or original contributor