gn build: Document llvm_enable_dia_sdk variable better

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359196 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/gn/secondary/llvm/lib/DebugInfo/PDB/enable_dia.gni b/utils/gn/secondary/llvm/lib/DebugInfo/PDB/enable_dia.gni
index 9d397bc..aed3566 100644
--- a/utils/gn/secondary/llvm/lib/DebugInfo/PDB/enable_dia.gni
+++ b/utils/gn/secondary/llvm/lib/DebugInfo/PDB/enable_dia.gni
@@ -1,4 +1,6 @@
 declare_args() {
   # Whether to build code that requires the Microsoft DIA SDK.
+  # If this is set, %INCLUDE% must contain ".../DIA SDK/include"
+  # and %LIB% must contain ".../DIA SKD/lib/amd64".
   llvm_enable_dia_sdk = false
 }