docs: document we don't alias for ONLY 1 OS[skip ci] (#2346)

Author: SteveLauC

CONVENTIONS.md

@@ -47,9 +47,13 @@ When creating newtypes, we use Rust's `CamelCase` type naming convention.
 ## cfg gates
 
 When creating operating-system-specific functionality, we gate it by
-`#[cfg(target_os = ...)]`. If more than one operating system is affected, we
+`#[cfg(target_os = ...)]`. If **MORE THAN ONE operating system** is affected, we
 prefer to use the cfg aliases defined in build.rs, like `#[cfg(bsd)]`.
 
+Please **DO NOT** use cfg aliases for **ONLY ONE** system as [they are bad][mismatched_target_os].
+
+[mismatched_target_os]: https://rust-lang.github.io/rust-clippy/master/index.html#/mismatched_target_os
+
 ## Bitflags
 
 Many C functions have flags parameters that are combined from constants using

build.rs

@@ -15,6 +15,8 @@ fn main() {
         watchos: { target_os = "watchos" },
         tvos: { target_os = "tvos" },
 
+
+        // cfg aliases we would like to use
         apple_targets: { any(ios, macos, watchos, tvos) },
         bsd: { any(freebsd, dragonfly, netbsd, openbsd, apple_targets) },
         linux_android: { any(android, linux) },