add objc_abi_version docs and move assert

Author: jbatez

compiler/rustc_codegen_llvm/src/context.rs

@@ -688,14 +688,17 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         llvm::set_section(g, c"llvm.metadata");
     }
 
+    /// The Objective-C ABI that is used.
+    ///
+    /// This corresponds to the `-fobjc-abi-version=` flag in Clang / GCC.
     pub(crate) fn objc_abi_version(&self) -> u32 {
+        assert!(self.tcx.sess.target.is_like_darwin);
         if self.tcx.sess.target.arch == "x86" && self.tcx.sess.target.os == "macos" {
             // 32-bit x86 macOS uses ABI version 1 (a.k.a. the "fragile ABI").
             1
         } else {
             // All other Darwin-like targets we support use ABI version 2
             // (a.k.a the "non-fragile ABI").
-            assert!(self.tcx.sess.target.is_like_darwin);
             2
         }
     }