doc: explain why passing -mlinker-version is required

Author: fanquake

depends/hosts/darwin.mk

@@ -5,6 +5,9 @@ XCODE_BUILD_ID=11C505
 LD64_VERSION=530
 
 OSX_SDK=$(SDK_PATH)/Xcode-$(XCODE_VERSION)-$(XCODE_BUILD_ID)-extracted-SDK-with-libcxx-headers
+
+# When cross-compiling for Darwin using Clang, -mlinker-version must be passed to
+# ensure that modern linker features are enabled.
 darwin_CC=clang -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) --sysroot $(OSX_SDK) -mlinker-version=$(LD64_VERSION)
 darwin_CXX=clang++ -target $(host) -mmacosx-version-min=$(OSX_MIN_VERSION) --sysroot $(OSX_SDK) -stdlib=libc++ -mlinker-version=$(LD64_VERSION)